Purine nucleoside analogues for treating Flaviviridae including hepatitis C

ABSTRACT

This invention is directed to a method for treating a host, especially a human, infected with hepatitis C, flavivirus and/or pestivirus, comprising administering to that host an effective amount of an anti-HCV biologically active pentofuranonucleoside where the pentofuranonucleoside base is an optionally substituted 2-azapurine. The optionally substituted pentofuranonucleoside, or a salt or prodrug thereof, may be administered alone or in combination with one or more optionally substituted pentofuranonucleosides or other anti-viral agents.

This application claims priority to U.S. Provisional Applicaton No.60/490,216, filed Jul. 25, 2003.

FIELD OF THE INVENTION

This invention is in the area of pharmaceutical chemistry, and, inparticular, in the area of purine nucleosides, their syntheses, andtheir use as anti-Flaviviridae agents in the treatment of hosts infectedwith Flaviviridae and especially with Hepatitis C.

BACKGROUND OF THE INVENTION

Flaviviridae Viruses

The Flaviviridae family of viruses comprises at least three distinctgenera: pestiviruses, which cause disease in cattle and pigs;flaviviruses, which are the primary cause of diseases such as denguefever and yellow fever; and hepaciviruses such as hepatitis C (HCV). Theflavivirus genus includes more than 68 members separated into groups onthe basis of serological relatedness (Calisher et al., J. Gen. Virol,1993, 70, 37-43). Clinical symptoms vary and include fever, encephalitisand hemorrhagic fever (Fields Virology, Editors: Fields, B. N., Knipe,D. M., and Howley, P. M., Lippincott-Raven Publishers, Philadelphia,Pa., 1996, Chapter 31, 931-959). Flaviviruses of global concern that areassociated with human disease include Dengue virus, hemorrhagic feverviruses such as Lassa, Ebola, and yellow fever virus, shock syndrome,and Japanese encephalitis virus (Halstead, S. B., Rev. Infect. Dis.,1984, 6, 251-264; Halstead, S. B., Science, 239:476-481, 1988; Monath,T. P., New Eng. J. Med., 1988, 319, 641-643).

The pestivirus genus includes bovine viral diarrhea virus (BVDV),classical swine fever virus (CSFV, also called hog cholera virus) andborder disease virus (BDV) of sheep (Moennig, V. et al. Adv. Vir. Res.1992, 41, 53-98). Pestivirus infections of domesticated livestock(cattle, pigs and sheep) cause significant economic losses worldwide.BVDV causes mucosal disease in cattle and is of significant economicimportance to the livestock industry (Meyers, G. and Thiel, H.-J.,Advances in Virus Research, 1996, 47, 53-118; Moennig V., et al, Adv.Vir. Res. 1992, 41, 53-98). Human pestiviruses have not been asextensively characterized as the animal pestiviruses. However,serological surveys indicate considerable pestivirus exposure in humans.

Pestiviruses and hepaciviruses are closely related virus groups withinthe Flaviviridae family. Other closely related viruses in this familyinclude the GB virus A, GB virus A-like agents, GB virus-B and GBvirus-C (also called hepatitis G virus, HGV). The hepacivirus group(hepatitis C virus; HCV) consists of a number of closely related butgenotypically distinguishable viruses that infect humans. There areapproximately 6 HCV genotypes and more than 50 subtypes. Due to thesimilarities between pestiviruses and hepaciviruses, combined with thepoor ability of hepaciviruses to grow efficiently in cell culture,bovine viral diarrhea virus (BVDV) is often used as a surrogate to studythe HCV virus.

The genetic organization of pestiviruses and hepaciviruses is verysimilar. These positive stranded RNA viruses possess a single large openreading frame (ORF) encoding all the viral proteins necessary for virusreplication. These proteins are expressed as a polyprotein that is co-and post-translationally processed by both cellular and virus-encodedproteinases to yield the mature viral proteins. The viral proteinsresponsible for the replication of the viral genome RNA are locatedwithin approximately the carboxy-terminal. Two-thirds of the ORF aretermed nonstructural (NS) proteins. The genetic organization andpolyprotein processing of the nonstructural protein portion of the ORFfor pestiviruses and hepaciviruses is very similar. For both thepestiviruses and hepaciviruses, the mature nonstructural (NS) proteins,in sequential order from the amino-terminus of the nonstructural proteincoding region to the carboxy-terminus of the ORF, consist of p7, NS1,NS2A, NS2B, NS3, NS4A, NS4B, NS5A, and NS5B.

The NS proteins of pestiviruses and hepaciviruses share sequence domainsthat are characteristic of specific protein functions. For example, theNS1 glycoprotein is a cell-surface protein that is translocated into theER lumen. NS1 was characterized initially as soluble complement-fixingantigen found in sera and tissues of infected animals, and now is knownto elicit humoral immune responses in its extracellular form. Antibodiesto NS1 may be used to confer passive immunity to certain pestivirusesand flaviviruses. NS1 has been implicated in the process of RNAreplication where it is believed to have a functional role in thecytoplasmic processing of RNA. NS2A is a small (approximately 22 kd)protein of unknown function. Studies suggest that it binds to NS3 andNS5, and so may be a recruiter of RNA templates to membrane-boundreplicase. NS2B also is a small (about 14 kd) protein that ismembrane-associated, and is a required cofactor for the serine proteasefunction of NS3, with which it forms a complex.

The NS3 proteins of viruses in both groups are large (about 70 kd),membrane-associated proteins that possess amino acid sequence motifscharacteristic of serine proteinases and of helicases (Gorbalenya et al.(1988) Nature 333:22; Bazan and Fletterick (1989) Virology 171:637-639;Gorbalenya et al. (1989) Nucleic Acid Res. 17.3889-3897). Thus, the NS3proteins have enzymatic activity needed for processing polyproteins forRNA replication. The C-terminal end of the NS3 proteins have an RNAtriphosphotase activity that appears to modify the 5′ end of the genomeprior to 5′-cap addition by guanylyltransferase.

NS4A and NS4B are membrane-associated, small (about 16 kd and about 27kd, respectively), hydrophobic proteins that appear to function in RNAreplication by anchoring replicase components to cellular membranes(Fields, Virology, 4^(th) Edition, 2001, p. 1001).

The NS5 proteins are the largest (about 103 kd) and most conserved, withsequence homology to other (+)-stranded RNA viruses. It also plays apivotal role in viral replication. The NS5B proteins of pestiviruses andhepaciviruses are the enzymes necessary for synthesis of thenegative-stranded RNA intermediate that is complementary to the viralgenome, and of the positive-stranded RNA that is complementary to thenegative-stranded RNA intermediate. The NS5B gene product hasGly-Asp-Asp (GDD) as a hallmark sequence, which it shares with reversetranscriptases and other viral polymerases and which is predictive ofRNA dependent RNA polymerase (RdRP) activity (DeFrancesco et al.,Antiviral Research, 2003, 58:1-16). Interestingly, it was found that theNS5B C-terminal 21 residue long hydrophobic tail is needed to targetNS5B to the ER membrane, but its removal has no other effect and, infact, leads to increased enzymatic solubility and activity (Tomei etal., J. Gen. Virol., 2000, 81:759-767; Lohmann et al., J. Virol., 1997,71:8416-28; Ferrari et al., J. Virol., 1999, 73:1649-54).

The NS5B enzyme products have the motifs characteristic of RNA-directedRNA polymerases, and in addition, share homology with methyltransferaseenzymes that are involved in RNA cap formation (Koonin, E. V. and Dolja,V. V. (1993) Crit. Rev. Biochem. Molec. Biol. 28:375-430; Behrens etal.(1996) EMBO J. 15:12-22; Lchmannet al.(1997) J. Virol. 71:8416-8428;Yuan et al.(1997) Biochem. Biophys. Res. Comm. 232:231-235; Hagedorn,PCT WO 97/12033; Zhong et al.(1998) J. Virol. 72.9365-9369). Theunliganded crystal structure of NS5B shows the unique structural featureof folding in a classic “right hand” shape, in which fingers, palm andthumb subdomains can be recognized (a feature it shares with otherpolymerases), but differs from other “half-open right hand” polymerasesby having a more compact shapes due to two extended loops that span thefinger and thumb domains at the top of the active site cavity(DeFrancesco et al. at 9). The finger, thumb and palm subdomainsencircle the active site cavity to which the RNA template and NTPsubstrates have access via two positively charged tunnels (Bressanelliet al., J. Virol., 2002, 76, 3482-92). Finger and thumb domains havestrong interactions that limit their ability to change conformationindependently of one another, a structural feature shared by otherRdRPs. The thumb domain contains a β-hairpin loop that extends towardthe cleft of the active site and may play a role in restricting thebinding of the template/primer at the enzyme active site (DeFrancesco etal., at 10). Studies are in progress to determine the role of this loopin the initiation mechanism of RNA synthesis (Id.)

Nucleotidyl transfer reaction residues are located in the palm domainand contain the signature GDD motif (DeFrancesco et al., at 9). Palmdomain geometry is highly conserved in all polymerases, and has aconserved two-metal-ion catalytic center that is required for catalyzinga phosphory transfer reaction at the polymerase active site.

It is believed that the de novo initiation model of RNA polymerization,rather than a “copy back” mechanism, is utilized by pesti-, flavi- andhepaciviruses. In the de novo initiation model, complementary RNAsynthesis is initiated at the 3′-end of the genome by a nucleotidetriphosphate rather than a nucleic acid or a protein primer. PurifiedNS5B is capable of this type of primer-independent action, and theC-terminal β-loop is believed to correctly position the 3′-end of theRNA template by functioning as a gate that retards slippage of the RNA3′-end through the polymerase active site (Hong et al., Virology, 2001,285:6-11. Bressanelli et al. reported the structure of NS5B polymerasein complex with nucleotides in which three distinct nucleotide-bindingsites were observed in the catalytic center of the HCV RdRP, and thecomplex exhibited a geometry similar to the de novo initiation complexof phi 6 polymerase (Bressanelli et al., J. Virol., 2002, 76: 3482-92).Thus, de novo initiation occurs and apparently is followed by RNAelongation, termination of polymerization, and release of the newstrand. At each of these steps is the opportunity for intervention andinhibition of the viral lifecycle.

The actual roles and functions of the NS proteins of pestiviruses andhepaciviruses in the lifecycle of the viruses are directly analogous. Inboth cases, the NS3 serine proteinase is responsible for all proteolyticprocessing of polyprotein precursors downstream of its position in theORF (Wiskerchen and Collett (1991) Virology 184:341-350; Bartenschlageret al. (1993) J. Virol. 67:3835-3844; Eckart et al. (1993) Biochem.Biophys. Res. Comm. 192:399-406; Grakoui et al. (1993) J. Virol.67:2832-2843; Grakoui et al. (1993) Proc. Natl. Acad. Sci. USA90:10583-10587; Hijikata et al. (1993) J. Virol. 67:4665-4675; Tome etal. (1993) J. Virol. 67:4017-4026). The NS4A protein, in both cases,acts as a cofactor with the NS3 serine protease (Bartenschlager et al.(1994) J. Virol. 68:5045-5055; Failla et al. (1994) J. Virol. 68:3753-3760; Lin et al. (1994) 68:8147-8157; Xu et al. (1997) J. Virol.71:5312-5322). The NS3 protein of both viruses also functions as ahelicase (Kim et al. (1995) Biochem. Biophys. Res. Comm. 215: 160-166;Jin and Peterson (1995) Arch. Biochem. Biophys., 323:47-53; Warrener andCollett (1995) J. Virol. 69:1720-1726). Finally, the NS5B proteins ofpestiviruses and hepaciviruses have the predicted RNA-directed RNApolymerases activity (Behrens et al.(1996) EMBO J. 15:12-22; Lchmannetal.(1997) J. Virol. 71:8416-8428; Yuan et al.(1997) Biochem. Biophys.Res. Comm. 232:231-235; Hagedorn, PCT WO 97/12033; Zhong et al.(1998) J.Virol. 72.9365-9369).

Hepatitis C Virus

The hepatitis C virus (HCV) is the leading cause of chronic liverdisease worldwide. (Boyer, N. et al. J. Hepatol. 32:98-112, 2000). HCVcauses a slow growing viral infection and is the major cause ofcirrhosis and hepatocellular carcinoma (Di Besceglie, A. M. and Bacon,B. R., Scientific American, Oct.: 80-85, (1999); Boyer, N. et al. J.Hepatol. 32:98-112, 2000). An estimated 170 million persons are infectedwith HCV worldwide. (Boyer, N. et al. J. Hepatol. 32:98-112, 2000).Cirrhosis caused by chronic hepatitis C infection accounts for8,000-12,000 deaths per year in the United States, and HCV infection isthe leading indication for liver transplantation.

HCV is known to cause at least 80% of posttransfusion hepatitis and asubstantial proportion of sporadic acute hepatitis. Preliminary evidencealso implicates HCV in many cases of “idiopathic” chronic hepatitis,“cryptogenic” cirrhosis, and probably hepatocellular carcinoma unrelatedto other hepatitis viruses, such as Hepatitis B Virus (HBV). A smallproportion of healthy persons appear to be chronic HCV carriers, varyingwith geography and other epidemiological factors. The numbers maysubstantially exceed those for HBV, though information is stillpreliminary; how many of these persons have subclinical chronic liverdisease is unclear. (The Merck Manual, ch. 69, p. 901, 16th ed.,(1992)).

HCV is an enveloped virus containing a positive-sense single-strandedRNA genome of approximately 9.4 kb. The viral genome consists of a 5′untranslated region (UTR), a long open reading frame encoding apolyprotein precursor of approximately 3011 amino acids, and a short 3′UTR. The 5′ UTR is the most highly conserved part of the HCV genome andis important for the initiation and control of polyprotein translation.Translation of the HCV genome is initiated by a cap-independentmechanism known as internal ribosome entry. This mechanism involves thebinding of ribosomes to an RNA sequence known as the internal ribosomeentry site (IRES). An RNA pseudoknot structure has recently beendetermined to be an essential structural element of the HCV IRES. Viralstructural proteins include a nucleocapsid core protein (C) and twoenvelope glycoproteins, E1 and E2.

HCV also encodes two proteinases, a zinc-dependent metalloproteinaseencoded by the NS2-NS3 region and a serine proteinase encoded in the NS3region. These proteinases are required for cleavage of specific regionsof the precursor polyprotein into mature peptides: the junction betweenNS2 and NS3 is autocatalytically cleaved the NS2/NS3 protease, while theremaining junctions are cleaved by the N-terminal serine protease domainof NS3 complexed with NS4A. The NS3 protein contains the NTP-dependenthelicase activity that unwinds duplex RNA during replication. Thehydrophobic carboxy-terminal 21 amino acids of nonstructural protein 5,NS5B, contains the RNA-dependent RNA polymerase that is essential forviral replication (Fields Virology, Fourth Edition, Editors: Fields, B.N., Knipe, D. M., and Howley, P. M., Lippincott-Raven Publishers,Philadelphia, Pa., 2001, Chapter 32, pp. 1014-1015). NS5B is known tobind RNAs nonspecifically, and to interact directly with NS3 and NS4Athat, in turn, form complexes with NS4B and NS5A (Id. @ 1015; Ishido etal., Biochem. Biophys. Res. Commun., 1998; 244:35-40). Certain in vitroexperiments using NS5B and guanosine 5′-mono-, di-, and triphosphate aswell as 5′-triphosphate of 2′-deoxy- and 2′,3′-dideoxy-guanosine as HCVinhibitors suggest that HCV-RdRP may have a strict specificity for5′-triphosphates and 2′- and 3′-OH groups (Watanabe et al., U.S.2002/0055483). Otherwise, the function(s) of the remaining nonstructuralproteins, NS4A, NS4B, and NS5A (the amino-terminal half of nonstructuralprotein 5) remain unknown.

A significant focus of current antiviral research is directed to thedevelopment of improved methods of treatment of chronic HCV infectionsin humans (Di Besceglie, A. M. and Bacon, B. R., Scientific American,Oct.: 80-85, (1999)).

Methods to Treat Flaviviridae Infections

The development of new antiviral agents for Flaviviridae infections,especially hepatitis C, is currently underway. Specific inhibitors ofHCV-derived enzymes such as protease, helicase, and polymeraseinhibitors are being developed. Drugs that inhibit other steps in HCVreplication are also in development, for example, drugs that blockproduction of HCV antigens from the RNA (IRES inhibitors), drugs thatprevent the normal processing of HCV proteins (inhibitors ofglycosylation), drugs that block entry of HCV into cells (by blockingits receptor) and nonspecific cytoprotective agents that block cellinjury caused by the virus infection. Further, molecular approaches arealso being developed to treat hepatitis C, for example, ribozymes, whichare enzymes that break down specific viral RNA molecules, and antisenseoligonucleotides, which are small complementary segments of DNA thatbind to viral RNA and inhibit viral replication, are underinvestigation. A number of HCV treatments are reviewed by Bymock et al.in Antiviral Chemistry & Chemotherapy, 11:2; 79-95 (2000) and DeFrancesco et al. in Antiviral Research, 58: 1-16 (2003).

Idenix Pharmaceuticals, Ltd. discloses branched nucleosides, and theiruse in the treatment of HCV and flaviviruses and pestiviruses in U.S.patent Publication Nos. 2003/0050229 A1, 2004/0097461 A1, 2004/0101535A1, 2003/0060400 A1, 2004/0102414 A1, 2004/0097462 A1, and 2004/0063622A1 which correspond to International Publication Nos. WO 01/90121 and WO01/92282. A method for the treatment of hepatitis C infection (andflaviviruses and pestiviruses) in humans and other host animals isdisclosed in the Idenix publications that includes administering aneffective amount of a biologically active 1′, 2′, 3′ or 4′-branched β-Dor β-L nucleosides or a pharmaceutically acceptable salt or prodrugthereof, administered either alone or in combination, optionally in apharmaceutically acceptable carrier. See also U.S. patent PublicationNos. 2004/0006002 and 2004/0006007 as well as WO 03/026589 and WO03/026675. Idenix Pharmaceuticals, Ltd. also discloses in U.S. patentPublication No. 2004/0077587 pharmaceutically acceptable branchednucleoside prodrugs, and their use in the treatment of HCV andflaviviruses and pestiviruses in prodrugs. See also PCT Publication Nos.WO 04/002422, WO 04/002999, and WO 04/003000. Further, IdenixPharmaceuticals, Ltd. also discloses in WO 04/046331 Flaviviridaemutations caused by biologically active 2′-branched β-D or β-Lnucleosides or a pharmaceutically acceptable salt or prodrug thereof.

Biota Inc. discloses various phosphate derivatives of nucleosides,including 1′, 2′, 3′ or 4′-branched β-D or β-L nucleosides, for thetreatment of hepatitis C infection in International Patent PublicationWO 03/072757.

Emory University and the University of Georgia Research Foundation, Inc.(UGARF) discloses the use of 2′-fluoronucleosides for the treatment ofHCV in U.S. Pat. No. 6,348,587. See also U.S. patent Publication No.2002/0198171 and International Patent Publication WO 99/43691.

BioChem Pharma Inc. (now Shire Biochem, Inc.) discloses the use ofvarious 1,3-dioxolane nucleosides for the treatment of a Flaviviridaeinfection in U.S. Pat. No. 6,566,365. See also U.S. Pat. Nos. 6,340,690and 6,605,614; U.S. patent Publication Nos. 2002/0099072 and2003/0225037, as well as International Publication No. WO 01/32153 andWO 00/50424.

BioChem Pharma Inc. (now Shire Biochem, Inc.) also discloses variousother 2′-halo, 2′-hydroxy and 2′-alkoxy nucleosides for the treatment ofa Flaviviridae infection in U.S. patent Publication No. 2002/0019363 aswell as International Publication No. WO 01/60315 (PCT/CA01/00197; filedFeb. 19, 2001).

ICN Pharmaceuticals, Inc. discloses various nucleoside analogs that areuseful in modulating immune response in U.S. Pat. Nos. 6,495,677 and6,573,248. See also WO 98/16184, WO 01/68663, and WO 02/03997.

U.S. Pat. No. 6,660,721; U.S. patent Publication Nos. 2003/083307 A1,2003/008841 A1, and 2004/0110718; as well as International PatentPublication Nos. WO 02/18404; WO 02/100415, WO 02/094289, and WO04/043159; filed by F. Hoffmann-La Roche A G, discloses variousnucleoside analogs for the treatment of HCV RNA replication.

Pharmasset Limited discloses various nucleosides and antimetabolites forthe treatment of a variety of viruses, including Flaviviridae, and inparticular HCV, in U.S. patent Publication Nos. 2003/0087873,2004/0067877, 2004/0082574, 2004/0067877, 2004/002479, 2003/0225029, and2002/00555483, as well as International Patent Publication Nos. WO02/32920, WO 01/79246, WO 02/48165, WO 03/068162, WO 03/068164 and WO2004/013298.

Merck & Co., Inc. and Isis Pharmaceuticals disclose in U.S. patentPublication No. 2002/0147160, 2004/0072788, 2004/0067901, and2004/0110717; as well as the corresponding International PatentPublication Nos. WO 02/057425 (PCT/US02/01531; filed Jan. 18, 2002) andWO 02/057287 (PCT/US02/03086; filed Jan. 18, 2002) various nucleosides,and in particular several pyrrolopyrimidine nucleosides, for thetreatment of viruses whose replication is dependent upon RNA-dependentRNA polymerase, including Flaviviridae, and in particular HCV. See alsoWO 2004/000858, WO 2004/003138, WO 2004/007512, and WO 2004/009020.

U.S. patent Publication No. 2003/028013 A1 as well as InternationalPatent Publication Nos. WO 03/051899, WO 03/061576, WO 03/062255 WO03/062256, WO 03/062257, and WO 03/061385, filed by Ribapharm, also aredirected to the use of certain nucleoside analogs to treat hepatitis Cvirus.

Genelabs Technologies disclose in U.S. patent Publication No.2004/0063658 as well as International Patent Publication Nos. WO03/093290 and WO 04/028481 various base modified derivatives ofnucleosides, including 1′, 2′, 3′ or 4′-branched β-D or β-L nucleosides,for the treatment of hepatitis C infection.

Eldrup et al. (Oral Session V, Hepatitis C Virus, Flaviviridae; 16^(th)International Conference on Antiviral Research (Apr. 27, 2003, Savannah,Ga.) p. A75) described the structure activity relationship of2′-modified nucleosides for inhibition of HCV.

Bhat et al (Oral Session V, Hepatitis C Virus, Flaviviridae; 16^(th)International Conference on Antiviral Research (Apr. 27, 2003, Savannah,Ga.); p A75) describe the synthesis and pharmacokinetic properties ofnucleoside analogues as possible inhibitors of HCV RNA replication. Theauthors report that 2′-modified nucleosides demonstrate potentinhibitory activity in cell-based replicon assays.

Olsen et al. (Oral Session V, Hepatitis C Virus, Flaviviridae; 16^(th)International Conference on Antiviral Research (Apr. 27, 2003, Savannah,Ga.) p A76) also described the effects of the 2′-modified nucleosides onHCV RNA replication.

Drug-resistant variants of viruses can emerge after prolonged treatmentwith an antiviral agent. Drug resistance most typically occurs bymutation of a gene that encodes for an enzyme used in viral replication,and, for example, in the case of HIV, reverse transcriptase, protease,or DNA polymerase. It has been demonstrated that the efficacy of a drugagainst viral infection can be prolonged, augmented, or restored byadministering the compound in combination or alternation with a second,and perhaps third, antiviral compound that induces a different mutationfrom that caused by the principle drug. Alternatively, thepharmacokinetics, biodistribution, or other parameter of the drug can bealtered by such combination or alternation therapy. In general,combination therapy is typically preferred over alternation therapybecause it induces multiple simultaneous pressures on the virus. Onecannot predict, however, what mutations will be induced in the viralgenome by a given drug, whether the mutation is permanent or transient,or how an infected cell with a mutated viral sequence will respond totherapy with other agents in combination or alternation. This isexacerbated by the fact that there is a paucity of data on the kineticsof drug resistance in long-term cell cultures treated with modernantiviral agents.

In view of the severity of diseases associated with pestiviruses,flaviviruses, and hepatitis C virus, and their pervasiveness in animalsand humans, it is an object of the present invention to provide acompound, method and composition for the treatment of a host infectedwith any member of the family Flaviviridae, including hepatitis C virus.

Further, it is an object of the present invention to provide a compound,method and pharmaceutically-acceptable composition for the prophylaxisand/or treatment of a host, and particularly a human, infected with anymember of the family Flaviviridae.

Further, given the rising threat of other Flaviviridae infections, thereremains a strong need to provide new effective pharmaceutical agentsthat have low toxicity to the host.

Therefore, it is an object of the present invention to provide acompound, method and composition for the treatment of a host infectedwith any member of the family Flaviviridae, including hepatitis C virus,that have low toxicity to the host.

It is another object of the present invention to provide a compound,method and composition generally for the treatment of patients infectedwith pestiviruses, flaviviruses, or hepaciviruses.

SUMMARY OF THE INVENTION

Methods and compositions for the treatment of pestivirus, flavivirus andhepatitis C virus infection are described that include administering aneffective amount of a beta-D or beta-L-nucleoside of the Formulae (I)and (II), or a pharmaceutically acceptable salt or prodrug thereof.

In a first principal embodiment, a compound of the Formula (I), or apharmaceutically acceptable salt or prodrug thereof, is provided:

wherein

-   -   each R is independently H, phosphate (including mono-, di-, or        triphosphate or a stabilized phosphate prodrug) or phosphonate;        optionally substituted alkyl including lower alkyl, optionally        substituted alkenyl or alkynyl, acyl, —C(O)-(alkyl), —C(O)(lower        alkyl), —C(O)-(alkenyl), —C(O)-(alkynyl), lipid, phospholipid,        carbohydrate, peptide, cholesterol, an amino acid residue or        derivative, or other pharmaceutically acceptable leaving group        that is capable of providing H or phosphate when administered in        vivo;    -   n is 0-2;    -   when X is CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,        C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,        CH—S-alkenyl, CH—S-alkynyl, CH-halogen, or C-(halogen)₂,    -   then each R¹ and R^(1′) is independently H, OH, optionally        substituted alkyl including lower alkyl, azido, cyano,        optionally substituted alkenyl or alkynyl, —C(O)O-(alkyl),        —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),        —O(acyl), —O(lower acyl), —O(alkyl), —O(lower alkyl),        —O(alkenyl), —O(alkynyl), halogen, halogenated alkyl, —NO₂,        —NH₂, —NH(lower alkyl), —N(lower alkyl)₂, —NH(acyl), —N(acyl)₂,        —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, S(O)N-alkyl,        S(O)N-alkenyl, S(O)N-alkynyl, or SCH-halogen, wherein alkyl,        alkenyl, and/or alkynyl may optionally be substituted;    -   when X is O, S[O]_(n), NH, N-alkyl, N-alkenyl, N-alkynyl,        S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, or SCH-halogen,    -   then each R¹ and R^(1′) is independently H, optionally        substituted alkyl including lower alkyl, azido, cyano,        optionally substituted alkenyl or alkynyl, —C(O)O-(alkyl),        —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),        halogenated alkyl, —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂,        —C(H)═N—NH₂, C(S)NH₂, C(S)NH(alkyl), or C(S)N(alkyl)₂, wherein        alkyl, alkenyl, and/or alkynyl may optionally be substituted;    -   each R² and R³ is independently H, OH, NH₂, SH, F, Cl, Br, I,        CN, NO₂, —C(O)NH₂, —C(O)NH(alkyl), and —C(O)N(alkyl)₂, N₃,        optionally substituted alkyl including lower alkyl, optionally        substituted alkenyl or alkynyl, halogenated alkyl,        —C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl),        —C(O)O-(alkynyl), —O(acyl), —O(alkyl), —O(alkenyl), —O(alkynyl),        —OC(O)NH₂, NC, C(O)OH, SCN, OCN, —S(alkyl), —S(alkenyl),        —S(alkynyl), —NH(alkyl), —N(alkyl)₂, —NH(alkenyl), —NH(alkynyl),        an amino acid residue or derivative, a prodrug or leaving group        that provides OH in vivo, or an optionally substituted 3-7        membered heterocyclic ring having O, S and/or N independently as        a heteroatom taken alone or in combination;    -   each R^(2′) and R^(3′) is independently H; optionally        substituted alkyl, alkenyl, or alkynyl; —C(O)O(alkyl),        —C(O)O(lower alkyl), —C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂,        —C(O)NH(alkyl), —C(O)N(alkyl)₂, —O(acyl), —O(lower acyl),        —O(alkyl), —O(lower alkyl), —O(alkenyl), halogen, halogenated        alkyl and particularly CF₃, azido, cyano, NO₂, —S(alkyl),        —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂,        —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂, and R₃ at        3′-C may also be OH; and    -   Base is selected from the group consisting of:        wherein:    -   each A independently is N or C—R⁵;    -   each W is H, OH, —O(acyl), —O(C₁₄ alkyl), —O(alkenyl),        —O(alkynyl), —OC(O)R⁴R⁴, —OC(O)N R⁴R⁴, SH, —S(acyl), —S(C₁₋₄        alkyl), NH₂, NH(acyl), N(acyl)₂, NH(C₁₋₄ alkyl), N(C₁₋₄ alkyl)₂,        —N(cycloalkyl) C₁₋₄ alkylamino, di(C₁I₄ alkyl)amino, C₃₋₆        cycloalkylamino, halogen, C₁₋₄ alkyl, C₁₋₄ alkoxy, CN, SCN, OCN,        SH, N₃, NO₂, NH═NH₂, N₃, NHOH, —C(O)NH₂, —C(O)NH(acyl),        —C(O)N(acyl)₂, —C(O)NH(C₁₋₄ alkyl), —C(O)N(C₁₋₄ alkyl)₂,        —C(O)N(alkyl)(acyl), or halogenated alkyl;    -   each Z is O, S, NH, N—OH, N—NH₂, NH(alkyl), N(alkyl)₂,        N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂, N₃, NH═NH,        NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂;    -   each R⁴ is independently H, acyl, or C₁₋₆ alkyl;    -   each R⁵ is independently H, Cl, Br, F, I, CN, OH, optionally        substituted alkyl, alkenyl or alkynyl, carboxy, C(═NH)NH₂, C₁₋₄        alkoxy, C₁₋₄ alkyloxycarbonyl, N₃, NH₂, NH(alkyl), N(alkyl)₂,        NO₂, N₃, halogenated alkyl especially CF₃, C₁₋₄ alkylamino,        di(C₁₋₄ alkyl)amino, C₃₋₆ cycloalkylamino, C₁₋₆ alkoxy, SH,        —S(C₁₋₄ alkyl), —S(C₁₋₄ alkenyl), —S(C₁₋₄ alkynyl), C₁₋₆        alkylthio, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl, C₃₋₆        cycloalkylamino-alkenyl, -alkynyl, —(O)alkyl, —(O)alkenyl,        —(O)alkynyl, —(O)acyl, —O(C₁₋₄ alkyl), —O(C₁₋₄ alkenyl), —O(C₁₋₄        alkynyl), —O—C(O)NH₂, —OC(O)N(alkyl), —OC(O)R′R″, —C(O)OH,        C(O)O-alkyl, C(O)O-alkenyl, C(O)O-alkynyl, S-alkyl, S-acyl,        S-alkenyl, S-alkynyl, SCN, OCN, NC, —C(O)—NH₂, C(O)NH(alkyl),        C(O)N(alkyl)₂, C(O)NH(acyl), C(O)N(acyl)₂, (S)—NH₂, NH-alkyl,        N(dialkyl)₂, NH-acyl, N-diacyl, or a 3-7 membered heterocycle        having O, S, or N taken independently in any combination;    -   each R′ and R″ independently is H, C₁₋₆ alkyl, C₂ ₆ alkenyl,        C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,        C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino,        C₁₋₆ alkoxy, C₁₋₆ alkylsulfonyl, or (C₁₋₄ alkyl)₀₋₂ aminomethyl;        and    -   all tautomeric, enantiomeric and stereoisomeric forms thereof;    -   with the caveat that when X is S in Formula (I), then the        compound is not        5-(4-amino-imidazo[4,5-d][1,2,3]triazin-7-yl)-2-hydroxymethyl-tetrahydro-thiophen-3-ol        or        7-(4-hydroxy-5-hydroxy-methyl-tetrahydro-thiophen-2-yl)-3,7-dihydro-imidazo[4,5-d][1,2,3]triazin-4-one.

In a second principal embodiment, a compound of the Formula (II), or apharmaceutically acceptable salt or prodrug thereof, is provided:

wherein:

-   -   R, R², R^(2′), R³, and R^(3′) are all as defined above;    -   X* is CY³;    -   Y³ is hydrogen, alkyl, bromo, chloro, fluoro, iodo, azido,        cyano, alkenyl, alkynyl, —C(O)O(alkyl), —C(O)O(lower alkyl),        CF₃, —CONH₂, —CONH(alkyl), or —CON(alkyl)₂;    -   R¹ is H, OH, optionally substituted alkyl including lower alkyl,        azido, cyano, optionally substituted alkenyl or alkynyl,        —C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl),        —C(O)O-(alkynyl), —O(acyl), —O(lower acyl), —O(alkyl), —O(lower        alkyl), —O(alkenyl), —O(alkynyl), halogen, halogenated alkyl,        —NO₂, —NH₂, —NH(lower alkyl), —N(lower alkyl)₂, —NH(acyl),        —N(acyl)₂, —C(O)NH₂, —C(O)NH(alkyl), or —C(O)N(alkyl)₂, wherein        an optional substitution on alkyl, alkenyl, and/or alkynyl may        be one or more halogen, hydroxy, alkoxy or alkylthio groups        taken in any combination;    -   Base is defined as above for formulae (A)-(G); and    -   all tautomeric, enantiomeric and stereoisomeric forms thereof;    -   with the caveat that when X is S in Formula (I), then the        compound is not        5-(4-amino-imidazo[4,5-d][1,2,3]triazin-7-yl)-2-hydroxymethyl-tetrahydro-thiophen-3-ol        or        7-(4-hydroxy-5-hydroxy-methyl-tetrahydro-thiophen-2-yl)-3,7-dihydro-imidazo[4,5-d][1,2,3]triazin-4-one.

In preferred embodiments, Bases (A)-(G) have a structure selected fromthe group consisting of:

wherein

-   -   each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,        C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,        CN₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino,        C₁₋₆ alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl, as        provided above in the definitions of A and Z for the Base        Formulae (A)-(G);    -   each W is independently H, Cl, Br, I, F, halogenated alkyl,        alkoxy, OH, SH, O-alkyl, S—alkyl, O-alkenyl, O-alkynyl,        S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl, CN, SCN, OCN,        NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl, NH-acyl,        NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and    -   each R⁴ is independently H, acyl, or C₁₋₆ alkyl;    -   each Z is O, S, NH, N—OH, N—NH₂, NH(alkyl), N(alkyl)₂,        N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂, N₃, NH═NH,        NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂.

In its preferred embodiments, the compounds of the present inventioncomprise nucleosides in which each variable in Formula (I) is selectedfrom the following, in any combination: X is O or S; R is H orphosphate; R₁ is H, CH₂OH, or CONH₂; R₂ is OH or F; R₃ is alkyl,especially methyl or propynyl, or H at the 3′ position; A is H, CH or N;Z is O, S, or NH; W is NH₂, Cl, OMe, OH, NH-cyclopropyl, S-Me; and eachR′ and R″ independently is Cl, CN, CONH₂ or Me.

In its preferred embodiments for Formula (II), the compounds of thepresent invention comprise nucleosides in which each variable in Formula(II) is selected from the following, in any combination: X* is CH; R isH or phosphate; R₁ is H, CH₂OH, or CONH₂; R₂ is OH or F; R₃ is alkyl,especially methyl or propynyl, or H at the 3′ position; A is H, CH or N;Z is O, S, or NH; W is NH₂, Cl, OMe, OH, NH-cyclopropyl, S-Me; and eachR′ and R″ independently is Cl, CN, CONH₂ or Me.

In all embodiments, optional substituents are selected from the groupconsisting of one or more halogen, amino, hydroxy, carboxy and alkoxygroups or atoms, among others. It is to be understood that allstereoisomeric and tautomeric forms of the compounds shown are includedherein.

The active compounds of the present invention can be administered incombination, alternation or sequential steps with another anti-HCVagent. In combination therapy, effective dosages of two or more agentsare administered together, whereas in alternation or sequential-steptherapy, an effective dosage of each agent is administered serially orsequentially. The dosages given will depend on absorption, inactivationand excretion rates of the drug as well as other factors known to thoseof skill in the art. It is to be noted that dosage values will also varywith the severity of the condition to be alleviated. It is to be furtherunderstood that for any particular subject, specific dosage regimens andschedules should be adjusted over time according to the individual needand the professional judgment of the person administering or supervisingthe administration of the compositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 show generalized structural depictions for Formula (I) andFormula (II) of the ribofuranosylnucleosides of the present invention.

FIG. 2 shows generalized structures for the 2-azapurine bases of thepresent invention.

FIG. 3 shows structural depictions for preferred bases of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a compound, method and composition forthe treatment of a pestivirus, flavivirus and/or hepatitis C in humansor other host animals that includes administering an effectiveanti-pestivirus, anti-flavivirus or anti-HCV treatment amount of abeta-D- or beta-L-nucleoside as described herein, or a pharmaceuticallyacceptable salt or prodrug thereof, optionally in a pharmaceuticallyacceptable carrier. The compounds of this invention either possessantiviral activity, or are metabolized to a compound that exhibits suchactivity.

Flaviviruses included within the scope of this invention are discussedgenerally in Fields Virology, Editors: Fields, N., Knipe, D. M. andHowley, P. M.; Lippincott-Raven Pulishers, Philadelphia, Pa.; Chapter 31(1996). Specific flaviviruses include, without limitation: Absettarov;Alfuy; Apoi; Aroa; Bagaza; Banzi; Bououi; Bussuquara; Cacipacore; CareyIsland; Dakar bat; Dengue viruses 1, 2, 3 and 4; Edge Hill; Entebbe bat;Gadgets Gully; Hanzalova; Hypr; Uheus; Israel turkeymeningoencephalitis; Japanese encephalitis; Jugra; Jutiapa; Kadam;Karshi; Kedougou; Kokoera; Koutango; Kumlinge; Kunjin; Kyasanur Forestdisease; Langat; Louping ill; Meaban; Modoc; Montana myotisleukoencephalitis; Murray valley encephalitis; Naranjal; Negishi; Ntaya;Omsk hemorrhagic fever; Phnom-Penh bat; Powassan; Rio Bravo; Rocio;Royal Farm; Russian spring-summer encephalitis; Saboya; St. Louisencephalitis; Sal Vieja; San Perlita; Saumarez Reef; Sepik; Sokuluk;Spondweni; Stratford; Temusu; Tyuleniy; Uganda S, Usutu, Wesselsbron;West Nile; Yaounde; Yellow fever; and Zika.

Pestiviruses included within the scope of this invention are alsodiscussed generally in Fields Virology (Id.). Specific pestivirusesinclude, without limitation: bovine viral diarrhea virus (“VDV”);classical swine fever virus (“CSFV”) also known as hog cholera virus);and border disease virus (“DV”).

HCV is a member of the family, Flaviviridae; however, HCV now has beenplaced in a new monotypic genus, hepacivirus.

Active Compounds, Pharmaceutically Acceptable Salts and Prodrugs Thereof

In a first principal embodiment, a compound of the Formula (I), or apharmaceutically acceptable salt or prodrug thereof, is provided:

wherein

-   -   R is H, phosphate (including mono-, di-, or triphosphate or a        stabilized phosphate prodrug) or phosphonate; optionally        substituted alkyl including lower alkyl, optionally substituted        alkenyl or alkynyl, acyl, —C(O)-(alkyl), —C(O)(lower alkyl),        —C(O)-(alkenyl), —C(O)-(alkynyl), lipid, phospholipid,        carbohydrate, peptide, cholesterol, an amino acid residue or        derivative, or other pharmaceutically acceptable leaving group        that is capable of providing H or phosphate when administered in        vivo;    -   n is 0-2;    -   when X is CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,        C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,        CH—S-alkenyl, CH—S-alkynyl, CH-halogen, or C-(halogen)₂,    -   then each R¹ and R^(1′) is independently H, OH, optionally        substituted alkyl including lower alkyl, azido, cyano,        optionally substituted alkenyl or alkynyl, —C(O)O-(alkyl),        —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),        —O(acyl), —O(lower acyl), —O(alkyl), —O(lower alkyl),        —O(alkenyl), —O(alkynyl), halogen, halogenated alkyl, —NO₂,        —NH₂, —NH(lower alkyl), —N(lower alkyl)₂, —NH(acyl), —N(acyl)₂,        —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, S(O)N-alkyl,        S(O)N-alkenyl, S(O)N-alkynyl, or SCH-halogen, wherein alkyl,        alkenyl, and/or alkynyl may optionally be substituted;    -   when X is O, S[O]_(n), NH, N-alkyl, N-alkenyl, N-alkynyl,        S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, or SCH-halogen,    -   then each R¹ and R^(1′) is independently H, optionally        substituted alkyl including lower alkyl, azido, cyano,        optionally substituted alkenyl or alkynyl, —C(O)O-(alkyl),        —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),        halogenated alkyl, —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂,        —C(H)═N—NH₂, C(S)NH₂, C(S)NH(alkyl), or C(S)N(alkyl)₂, wherein        alkyl, alkenyl, and/or alkynyl may optionally be substituted;    -   each R² and R³ is independently is OH, NH₂, SH, F, Cl, Br, I,        CN, NO₂, —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, N₃,        optionally substituted alkyl including lower alkyl, optionally        substituted alkenyl or alkynyl, halogenated alkyl,        —C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl),        —C(O)O-(alkynyl), —O(acyl), —O(alkyl), —O(alkenyl), —O(alkynyl),        —OC(O)NH₂, NC, C(O)OH, SCN, OCN, —S(alkyl), —S(alkenyl),        —S(alkynyl), —NH(alkyl), —N(alkyl)₂, —NH(alkenyl), —NH(alkynyl),        an amino acid residue or derivative, a prodrug or leaving group        that provides OH in vivo, or an optionally substituted 3-7        membered heterocyclic ring having O, S and/or N independently as        a heteroatom taken alone or in combination;    -   each R^(2′) and R^(3′) independently is H; optionally        substituted alkyl, alkenyl, or alkynyl; —C(O)O(alkyl),        —C(O)O(lower alkyl), —C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂,        —C(O)NH(alkyl), —C(O)N(alkyl)₂, —O(acyl), —O(lower acyl),        —O(alkyl), —O(lower alkyl), —O(alkenyl), halogen, halogenated        alkyl and particularly CF₃, azido, cyano, NO₂, —S(alkyl),        —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂,        —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂, and R₃ at        3′-C may also be OH; and    -   Base is selected from the group consisting of:        wherein    -   each A independently is N or C—R⁵;    -   W is H, OH, —O(acyl), —O(C₁₋₄ alkyl), —O(alkenyl), —O(alkynyl),        —OC(O)R⁴R⁴, —OC(O)N R⁴R⁴, SH, —S(acyl), —S(C₁₋₄ alkyl), NH₂,        NH(acyl), N(acyl)₂, NH(C₁₋₄ alkyl), N(C₁₋₄ alkyl)₂,        —N(cycloalkyl) C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₃₋₆        cycloalkylamino, halogen, C₁₋₄ alkyl, C₁₋₄ alkoxy, CN, SCN, OCN,        SH, N₃, NO₂, NH═NH₂, N₃, NHOH, —C(O)NH₂, —C(O)NH(acyl),        —C(O)N(acyl)₂, —C(O)NH(C₁₋₄ alkyl), —C(O)N(C₁₋₄ alkyl)₂,        —C(O)N(alkyl)(acyl), or halogenated alkyl;    -   Z is O, S, NH, N—OH, N—NH₂, NH(alkyl), N(alkyl)₂, N-cycloalkyl,        alkoxy, CN, SCN, OCN, SH, NO₂, NH₂, N₃, NH═NH, NH(alkyl),        N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂;    -   each R⁴ is independently H, acyl, or C₁₋₆ alkyl;    -   each R⁵ is independently H, Cl, Br, F, I, CN, OH, optionally        substituted alkyl, alkenyl or alkynyl, carboxy, C(═NH)NH₂, C₁₋₄        alkoxy, C₁₋₄ alkyloxycarbonyl, N₃, NH₂, NH(alkyl), N(alkyl)₂,        NO₂, N₃, halogenated alkyl especially CF₃, C₁₋₄ alkylamino,        di(C₁₋₄ alkyl)amino, C₃₋₆ cycloalkylamino, C₁₋₆ alkoxy, SH,        -$(C₁₋₄ alkyl), —S(C₁₋₄ alkenyl), —S(C₁₋₄ alkynyl), C₁₋₆        alkylthio, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl, C₃₋₆        cycloalkylamino-alkenyl, -alkynyl, —(O)alkyl, —(O)alkenyl,        —(O)alkynyl, —(O)acyl, —O(C₁₋₄ alkyl), —O(C₁₋₄ alkenyl), —O(C₁₋₄        alkynyl), —O—C(O)NH₂, —OC(O)N(alkyl), —OC(O)R′R″, —C(O)OH,        C(O)O-alkyl, C(O)O-alkenyl, C(O)O-alkynyl, S-alkyl, S-acyl,        S-alkenyl, S-alkynyl, SCN, OCN, NC, —C(O)—NH₂, C(O)NH(alkyl),        C(O)N(alkyl)₂, C(O)NH(acyl), C(O)N(acyl)₂, (S)—NH₂, NH-alkyl,        N(dialkyl)₂, NH-acyl, N-diacyl, or a 3-7 membered heterocycle        having O, S, or N taken independently in any combination;    -   each R′ and R″ independently is H, C₁₋₆ alkyl, C₂6 alkenyl, C₂₋₆        alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,        C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino,        C₁₋₆ alkoxy, C₁₋₆ alkylsulfonyl, or (C₁₋₄ alkyl)₀₋₂ aminomethyl;        and    -   all tautomeric, enantiomeric and stereoisomeric forms thereof;    -   with the caveat that when X is S in Formula (I), then the        compound is not        5-(4-amino-imidazo[4,5-d][1,2,3]triazin-7-yl)-2-hydroxymethyl-tetrahydro-thiophen-3-ol        or        7-(4-hydroxy-5-hydroxy-methyl-tetrahydro-thiophen-2-yl)-3,7-dihydro-imidazo[4,5-d][1,2,3]triazin-4-one.

In a second principal embodiment, a compound of the Formula (II), or apharmaceutically acceptable salt or prodrug thereof, is provided:

wherein:

-   -   R, R², R^(2′), R³, and R^(3′) are all as defined above;    -   X* is CY³;    -   Y³ is hydrogen, alkyl, bromo, chloro, fluoro, iodo, azido,        cyano, alkenyl, alkynyl, —C(O)O(alkyl), —C(O)O(lower alkyl),        CF₃, —CONH₂, —CONH(alkyl), or —CON(alkyl)₂;    -   R¹ is H, OH, optionally substituted alkyl including lower alkyl,        azido, cyano, optionally substituted alkenyl or alkynyl,        —C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl),        —C(O)O-(alkynyl), —O(acyl), —O(lower acyl), —O(alkyl), —O(lower        alkyl), —O(alkenyl), —O(alkynyl), halogen, halogenated alkyl,        —NO₂, —NH₂, —NH(lower alkyl), —N(lower alkyl)₂, —NH(acyl),        —N(acyl)₂, —C(O)NH₂, —C(O)NH(alkyl), or —C(O)N(alkyl)₂, wherein        an optional substitution on alkyl, alkenyl, and/or alkynyl may        be one or more halogen, hydroxy, alkoxy or alkylthio groups        taken in any combination;    -   Base is defined as above for formulae (A)-(G); and    -   A and Z are as defined above,    -   with the caveat that when X is S in Formula (I), then the        compound is not        5-(4-amino-imidazo[4,5-d][1,2,3]triazin-7-yl)-2-hydroxymethyl-tetrahydro-thiophen-3-ol        or        7-(4-hydroxy-5-hydroxy-methyl-tetrahydro-thiophen-2-yl)-3,7-dihydro-imidazo[4,5-d][1,2,3]triazin-4-one;        and    -   all tautomeric, enantiomeric and stereoisomeric forms thereof.

In preferred embodiments, Bases (A)-(G) have a structure selected fromthe group consisting of:

wherein

-   -   each R′ and R″ independently is H, C₁₋₆ alkyl, C₂6 alkenyl, C₂₋₆        alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,        C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino,        C₁₋₆ alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl, as        provided above in the definitions of A and Z for the Base        Formulae (A)-(G);    -   each W is Cl, Br, I, F, halogenated alkyl, alkoxy, OH, SH,        O-alkyl, S-alkyl, O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl,        —OC(O)NR⁴R⁴, O-acyl, S-acyl, CN, SCN, OCN, NO₂, N₃, NH₂,        NH(alkyl), N(alkyl)₂, NH-cycloalkyl, NH-acyl, NH═NH, CONH₂,        CONH(alkyl), or CON(alkyl)₂;    -   each R⁴ is independently H, acyl, or C₁₋₆ alkyl; and    -   each Z is O, S, NH, N—OH, N—NH₂, NH(alkyl), N(alkyl)₂,        N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂, N₃, NH═NH,        NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂.

In its preferred embodiments, the compounds of the present inventioncomprise nucleosides in which each variable in Formula (I) is selectedfrom the following, in any combination: X is O or S; R is H orphosphate; R₁ is H, CH₂OH, or CONH₂; R₂ is OH or F; R₃ is alkyl,especially methyl or propynyl, or H at the 3′ position; A is H, CH or N;Z is O, S, or NH; W is NH₂, Cl, OMe, OH, NH-cyclopropyl, S-Me; and eachR′ and R″ independently is Cl, CN, CONH₂ or Me.

In its preferred embodiments for Formula (II), the compounds of thepresent invention comprise nucleosides in which each variable in Formula(II) is selected from the following, in any combination: X* is CH; R isH or phosphate; R₁ is H, CH₂OH, or CONH₂; R₂ is OH or F; R₃ is alkyl,especially methyl or propynyl, or H at the 3′ position; A is H, CH or N;Z is O, S, or NH; W is NH₂, Cl, OMe, OH, NH-cyclopropyl, S-Me; and eachR′ and R″ independently is Cl, CN, CONH₂ or Me.

In all embodiments, optional substituents are selected from the groupconsisting of one or more halogen, amino, hydroxy, carboxy and alkoxygroups or atoms, among others. It is to be understood that allstereoisomeric and tautomeric forms of the compounds shown are includedherein.

In one particular embodiment, a compound of the Formula (III), or apharmaceutically acceptable salt or prodrug thereof, is provided:

-   -   each R, R²*, and R³* independently is H, phosphate (including        mono-, di-, or triphosphate or a stabilized phosphate prodrug)        or phosphonate; optionally substituted alkyl including lower        alkyl, optionally substituted alkenyl or alkynyl, acyl,        —C(O)-(alkyl), —C(O)(lower alkyl), —C(O)-(alkenyl),        —C(O)-(alkynyl), lipid, phospholipid, carbohydrate, peptide,        cholesterol, an amino acid residue or derivative, or other        pharmaceutically acceptable leaving group that is capable of        providing H or phosphate when administered in vivo;    -   X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,        C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,        CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,        S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or        C-(halogen)₂, wherein alkyl, alkenyl or alkynyl optionally may        be substituted;    -   n is 0-2;    -   each R^(2′) independently is H; optionally substituted alkyl,        alkenyl, or alkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl),        —C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl),        —C(O)N(alkyl)₂, —OH, —O(acyl), —O(lower acyl), —O(alkyl),        —O(lower alkyl), —O(alkenyl), halogen, halogenated alkyl and        particularly CF₃, azido, cyano, NO₂, —S(alkyl), —S(alkenyl),        —S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂, —NH(alkenyl),        —NH(alkynyl), —NH(acyl), or —N(acyl)₂; and    -   Base is defined as above for formulae (A)-(G); and preferably is        a Base as defined by structures (i)-(xi) above.

In one embodiment, the R^(2′) is an optionally substituted alkyl,alkenyl, or alkynyl; halogen, halogenated alkyl and particularly CF₃,azido, or cyano. In a particular embodiment, R^(2′) is an optionallysubstituted alkyl, alkenyl, or alkynyl; halogen, halogenated alkyl andparticularly CF₃. In yet another particular embodiment, R^(2′) is CH₃ orCF₃.

In one embodiment, each R, R²*, and R³* is independently H, phosphate(including mono-, di-, or triphosphate or a stabilized phosphateprodrug) or phosphonate. In anther embodiment, each R, R²*, and R³* isindependently H. In yet another embodiment, each R, R²*, and R³* isindependently H, acyl, or an amino acid acyl residue.

In one embodiment, X is O or S. In another embodiment, X is O.

In another particular embodiment, a compound of the Formula (IV), or apharmaceutically acceptable salt or prodrug thereof, is provided:

-   -   each R, R²*, and R³* independently is H, phosphate (including        mono-, di-, or triphosphate or a stabilized phosphate prodrug)        or phosphonate; optionally substituted alkyl including lower        alkyl, optionally substituted alkenyl or alkynyl, acyl,        —C(O)-(alkyl), —C(O)(lower alkyl), —C(O)-(alkenyl),        —C(O)-(alkynyl), lipid, phospholipid, carbohydrate, peptide,        cholesterol, an amino acid residue or derivative, or other        pharmaceutically acceptable leaving group that is capable of        providing H or phosphate when administered in vivo;    -   X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,        C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,        CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,        S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or        C-(halogen)₂, wherein alkyl, alkenyl or alkynyl optionally may        be substituted;    -   n is 0-2;    -   each R^(3′) independently is H; optionally substituted alkyl,        alkenyl, or alkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl),        —C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl),        —C(O)N(alkyl)₂, —OH, —O(acyl), —O(lower acyl), —O(alkyl),        —O(lower alkyl), —O(alkenyl), halogen, halogenated alkyl and        particularly CF₃, azido, cyano, NO₂, —S(alkyl), —S(alkenyl),        —S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂, —NH(alkenyl),        —NH(alkynyl), —NH(acyl), or —N(acyl)₂; and    -   Base is defined as above for formulae (A)-(G); and preferably is        a Base as defined by structures (i)-(xi) above.

In one embodiment, the R^(3′) is an optionally substituted alkyl,alkenyl, or alkynyl; halogen, halogenated alkyl and particularly CF₃,azido, or cyano. In a particular embodiment, R^(3′) is an optionallysubstituted alkyl, alkenyl, or alkynyl; halogen, halogenated alkyl andparticularly CF₃. In yet another particular embodiment, R^(3′) is CH₃ orCF₃.

In one embodiment, each R, R²*, and R³* is independently H, phosphate(including mono-, di-, or triphosphate or a stabilized phosphateprodrug) or phosphonate.

In anther embodiment, each R, R²*, and R³* is independently H. In yetanother embodiment, each R, R²*, and R³* is independently H, acyl, or anamino acid acyl residue.

In one embodiment, X is O or S. In another embodiment, X is O.

The beta-D- and beta-L-nucleosides of this invention belong to a classof anti-pestivirus, anti-flavivirus and anti-HCV agents that inhibitviral polymerase. Triphosphate nucleosides can be screened for theirability to inhibit viral polymerase, whether HCV, flavivirus orpestivirus, in vitro according to screening methods set forth below.Chiron Corporation developed a replicon system for testing potentialanti-HCV compounds that utilizes a particular peptide sequence having anHCV protease-recognition site (U.S. Pat. No. 6,436,666; U.S. Pat. No.6,416,946; U.S. Pat. No. 6,416,944; U.S. Pat. No. 6,379,886; and U.S.Pat. No. 6,326,151, to Chiron Corporation). Other systems for assessingthe ability of compounds to inhibit HCV and related viruses includethose of Rice (see U.S. Pat. No. 5,874,565) and the polymeraseinhibition assay of Dr. Ralf Bartenschlager (see EP 1 043 399 A2).

An alternative means of assessing a compound's ability to inhibit HCV,pestivirus and/or flavivirus is through the use of predictive animalmodel systems. The model of choice for testing HCV is the chimpanzee,which has been used by the applicants. Chimpanzees provide an excellentmammalian system for study of anti-HCV compounds and an insight into thepredictability or unpredictability of drug activity based on thecloseness of their species relationship to humans.

The active compounds of the present invention can be administered incombination, alternation or sequential steps with another anti-HCVagent. In combination therapy, effective dosages of two or more agentsare administered together, whereas in alternation or sequential-steptherapy, an effective dosage of each agent is administered serially orsequentially. The dosages given will depend on absorption, inactivationand excretion rates of the drug as well as other factors known to thoseof skill in the art. It is to be noted that dosage values will also varywith the severity of the condition to be alleviated. It is to be furtherunderstood that for any particular subject, specific dosage regimens andschedules should be adjusted over time according to the individual needand the professional judgment of the person administering or supervisingthe administration of the compositions.

In particular, the present invention provides the following:

-   -   a) a beta-D- or beta-L-nucleoside compound of Formula (I)-(IV),        or a pharmaceutically acceptable salt or prodrug thereof;    -   b) a pharmaceutical composition comprising a beta-D- or        beta-L-nucleoside compound of Formula (I)-(IV), or a        pharmaceutically acceptable salt or prodrug thereof, optionally        together with a pharmaceutically acceptable carrier, excipient        or diluent;    -   c) a pharmaceutical composition comprising a beta-D- or        beta-L-nucleoside compound of Formula (I)-(IV), or a        pharmaceutically acceptable salt or prodrug thereof, with one or        more other effective antiviral agents, optionally with a        pharmaceutically acceptable carrier or diluent;    -   d) a pharmaceutical composition for the treatment or prophylaxis        of a pestivirus, flavivirus or HCV infection in a host,        especially a host diagnosed as having or being at risk for such        infection, comprising a beta-D- or beta-L-nucleoside compound of        Formula (I)-(IV), or a pharmaceutically acceptable salt or        prodrug thereof, together with a pharmaceutically acceptable        carrier or diluent;    -   e) a pharmaceutical formulation comprising the beta-D- or        beta-L-nucleoside compound of Formula (I)-(IV), or a        pharmaceutically acceptable salt or prodrug thereof, together        with a pharmaceutically acceptable carrier, excipient or        diluent;    -   f) a method for the treatment of a pestivirus, flavivirus or HCV        infection in a host comprising a beta-D- or beta-L-nucleoside        compound of Formula (I)-(IV), or a pharmaceutically acceptable        salt or prodrug thereof, optionally with a pharmaceutically        acceptable carrier, excipient or diluent;    -   g) a method for the treatment of a pestivirus, flavivirus or HCV        infection in a host comprising administering an effective amount        of a beta-D- or beta-L-nucleoside compound of Formula (I)-(IV),        or a pharmaceutically acceptable salt or prodrug thereof, with        one or more other effective antiviral agents, optionally with a        pharmaceutically acceptable carrier, excipient or diluent;    -   h) a method for the treatment of a pestivirus, flavivirus or HCV        infection in a host comprising administering an effective amount        of a beta-D- or beta-L-nucleoside compound of Formula (I)-(IV),        or a pharmaceutically acceptable salt or prodrug thereof, with        one or more other effective antiviral agents, optionally with a        pharmaceutically acceptable carrier, excipient or diluent;    -   i) a method for the treatment of a pestivirus, flavivirus or HCV        infection in a host comprising administering an effective amount        of a beta-D- or beta-L-nucleoside compound of Formula (I)-(IV),        or a pharmaceutically acceptable salt or prodrug thereof, with        one or more other effective antiviral agents, optionally with a        pharmaceutically acceptable carrier, excipient or diluent;    -   j) a method for the treatment of a pestivirus, flavivirus or HCV        infection in a host comprising administering an effective amount        of a beta-D- or beta-L-nucleoside compound of Formula (I)-(IV),        or a pharmaceutically acceptable salt or prodrug thereof, with        one or more other effective antiviral agents, optionally with a        pharmaceutically acceptable carrier, excipient or diluent;    -   k) use of a beta-D- or beta-L-nucleoside compound of Formula        (I)-(IV), or a pharmaceutically acceptable salt or prodrug        thereof, optionally with a pharmaceutically acceptable carrier        or diluent, for the treatment of a pestivirus, flavivirus or HCV        infection in a host;    -   l) use of a beta-D- or beta-L-nucleoside compound of Formula        (I)-(IV), or a pharmaceutically acceptable salt or prodrug        thereof, with one or more other effective antiviral agents,        optionally with a pharmaceutically acceptable carrier or        diluent, for the treatment of a pestivirus, flavivirus and/or        HCV infection in a host;    -   m) use of a beta-D- or beta-L-nucleoside compound of Formula        (I)-(IV), or a pharmaceutically acceptable salt or prodrug        thereof, optionally with a pharmaceutically acceptable carrier        or diluent, in the manufacture of a medicament for the treatment        of a pestivirus, flavivirus and/or HCV infection in a host;    -   n) use of a beta-D- or beta-L-nucleoside compound of Formula        (I)-(IV), or a pharmaceutically acceptable salt or prodrug        thereof, with one or more other effective antiviral agents and        optionally with a pharmaceutically acceptable carrier, excipient        or diluent, in the manufacture of a medicament for the treatment        of a pestivirus, flavivirus and/or HCV infection in a host;    -   o) a beta-D- or beta-L-nucleoside compound of Formula (I)-(IV),        or a pharmaceutically acceptable salt or prodrug thereof,        substantially in the absence of enantiomers of the described        nucleoside, or substantially isolated from other chemical        entities;    -   p) a process for the preparation of a beta-D- or        beta-L-nucleoside compound of Formula (I)-(IV), or a        pharmaceutically acceptable salt or prodrug thereof, as provided        in more detail below; and    -   q) a process for the preparation of a beta-D- or        beta-L-nucleoside compound of Formula (I)-(IV), or a        pharmaceutically acceptable salt or prodrug thereof,        substantially in the absence of enantiomers of the described        nucleoside or substantially isolated from other chemical        entities.

The active compound can be administered as any salt or prodrug that uponadministration to the recipient is capable of providing directly orindirectly the parent compound, or that exhibits activity itself.Non-limiting examples are the pharmaceutically acceptable salts, whichare alternatively referred to as “physiologically acceptable salts”, anda compound that has been alkylated or acylated at the 5′-position or onthe purine or pyrimidine base, thereby forming a type of“pharmaceutically acceptable prodrug”. Further, the modifications canaffect the biological activity of the compound, in some cases increasingthe activity over the parent compound. This can easily be assessed bypreparing the salt or prodrug and testing its antiviral activityaccording to the methods described herein, or other methods known tothose skilled in the art.

Stereochemistry

It is appreciated that nucleosides of the present invention have severalchiral centers and may exist in and be isolated in optically active andracemic forms. Some compounds may exhibit polymorphism. It is to beunderstood that the present invention encompasses any racemic,optically-active, diastereomeric, polymorphic, or stereoisomeric form,or mixtures thereof, of a compound of the invention, which possess theuseful properties described herein. It being well known in the art howto prepare optically active forms (for example, by resolution of theracemic form by recrystallization techniques, by synthesis fromoptically-active starting materials, by chiral synthesis, or bychromatographic separation using a chiral stationary phase).

Examples of methods to obtain optically active materials are known inthe art, and include at least the following.

-   -   i) physical separation of crystals—a technique whereby        macroscopic crystals of the individual enantiomers are manually        separated. This technique can be used if crystals of the        separate enantiomers exist, i.e., the material is a        conglomerate, and the crystals are visually distinct;    -   ii) simultaneous crystallization—a technique whereby the        individual enantiomers are separately crystallized from a        solution of the racemate, possible only if the latter is a        conglomerate in the solid state;    -   iii) enzymatic resolutions—a technique whereby partial or        complete separation of a racemate by virtue of differing rates        of reaction for the enantiomers with an enzyme;    -   iv) enzymatic asymmetric synthesis—a synthetic technique whereby        at least one step of the synthesis uses an enzymatic reaction to        obtain an enantiomerically pure or enriched synthetic precursor        of the desired enantiomer;    -   v) chemical asymmetric synthesis—a synthetic technique whereby        the desired enantiomer is synthesized from an achiral precursor        under conditions that produce asymmetry (i.e., chirality) in the        product, which may be achieved using chiral catalysts or chiral        auxiliaries;    -   vi) diastereomer separations—a technique whereby a racemic        compound is reacted with an enantiomerically pure reagent (the        chiral auxiliary) that converts the individual enantiomers to        diastereomers. The resulting diastereomers are then separated by        chromatography or crystallization by virtue of their now more        distinct structural differences and the chiral auxiliary later        removed to obtain the desired enantiomer;    -   vii) first- and second-order asymmetric transformations—a        technique whereby diastereomers from the racemate equilibrate to        yield a preponderance in solution of the diastereomer from the        desired enantiomer or where preferential crystallization of the        diastereomer from the desired enantiomer perturbs the        equilibrium such that eventually in principle all the material        is converted to the crystalline diastereomer from the desired        enantiomer. The desired enantiomer is then released from the        diastereomer;    -   viii) kinetic resolutions—this technique refers to the        achievement of partial or complete resolution of a racemate (or        of a further resolution of a partially resolved compound) by        virtue of unequal reaction rates of the enantiomers with a        chiral, non-racemic reagent or catalyst under kinetic        conditions;    -   ix) enantiospecific synthesis from non-racernic precursors—a        synthetic technique whereby the desired enantiomer is obtained        from non-chiral starting materials and where the stereochemical        integrity is not or is only minimally compromised over the        course of the synthesis;    -   x) chiral liquid chromatography—a technique whereby the        enantiomers of a racemate are separated in a liquid mobile phase        by virtue of their differing interactions with a stationary        phase. The stationary phase can be made of chiral material or        the mobile phase can contain an additional chiral material to        provoke the differing interactions;    -   xi) chiral gas chromatography—a technique whereby the racemate        is volatilized and enantiomers are separated by virtue of their        differing interactions in the gaseous mobile phase with a column        containing a fixed non-racemic chiral adsorbent phase;    -   xii) extraction with chiral solvents—a technique whereby the        enantiomers are separated by virtue of preferential dissolution        of one enantiomer into a particular chiral solvent;    -   xiii) transport across chiral membranes—a technique whereby a        racemate is placed in contact with a thin membrane barrier. The        barrier typically separates two miscible fluids, one containing        the racemate, and a driving force such as concentration or        pressure differential causes preferential transport across the        membrane barrier. Separation occurs as a result of the        non-racemic chiral nature of the membrane which allows only one        enantiomer of the racemate to pass through.        Definitions

The term “alkyl” as used herein, unless otherwise specified, refers to asaturated straight, branched, or cyclic, primary, secondary, or tertiaryhydrocarbon of typically C₁ to C₁₀, and specifically includes methyl,trifluoromethyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl,t-butyl, pentyl, cyclopentyl, isopentyl, neopentyl, hexyl, isohexyl,cyclohexyl, cyclohexylmethyl, 3-methylpentyl, 2,2-dimethybutyl, and2,3-dimethylbutyl. The term includes both substituted and unsubstitutedalkyl groups. Moieties with which the alkyl group can be substitutedwith one or more substituents are selected from the group consisting ofhalo, including Cl, F, Br and I so as to form, for eg., CF₃, 2-Br-ethyl,CH₂F, CH₂Cl, CH₂CF₃, or CF₂CF₃; hydroxyl, for eg. CH₂OH; amino, for eg.,CH₂NH₂, CH₂NHCH₃, or CH₂N(CH₃)₂; carboxylate; carboxamido; alkylamino;arylamino; alkoxy; aryloxy; nitro; azido, for eg., CH₂N₃; cyano, foreg., CH₂CN; thio; sulfonic acid; sulfate; phosphonic acid; phosphate;and phosphonate, either unprotected or protected as necessary, known tothose skilled in the art, for eg., as taught in Greene et al.,Protective Groups in Organic Synthesis, John Wiley and Sons, SecondEdition (1991), incorporated herein by reference.

The term “lower alkyl” as used herein, and unless otherwise specified,refers to a C₁ to C₆ saturated straight, branched, or if appropriate,cyclic as in cyclopropyl, for eg., alkyl group, including bothsubstituted and unsubstituted forms. Unless otherwise specificallystated in this application, when alkyl is a suitable moiety, lower alkylis preferred. Similarly, when alkyl or lower alkyl is a suitable moiety,unsubstituted alkyl or lower alkyl is preferred.

The terms “alkylamino” and “arylamino” refer to an amino group that hasone or two alkyl or aryl substituents, respectively.

The term “protected” as used herein and, unless otherwise defined,refers to a group that is added to an oxygen, nitrogen or phosphorusatom to prevent its further reaction or for other purposes. Numerousoxygen and nitrogen protecting groups are known to those skilled in theart of organic synthesis.

The term “aryl” as used herein and, unless otherwise specified, refersto phenyl, biphenyl or naphthyl, and preferably phenyl. The termincludes both substituted and unsubstituted moieties. The aryl group canbe substituted with one or more moieties selected from the groupconsisting of alkyl, hydroxyl, amino, alkylamino, arylamino, alkoxy,aryloxy, nitro, cyano, thio, alkylthio, carboxamido, carboxylate,sulfonic acid, sulfate, phosphonic acid, phosphate, or phosphonate,either unprotected or protected as necessary, as known to those skilledin the art, for eg., as taught in Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition (1991),incorporated herein by reference.

The terms “alkaryl” and “akylaryl” refer to an alkyl group with an arylsustituent.

The terms “aralkyl” and “arylalkyl” refer to an aryl group with an alkylsubstituent.

The term “halo” as used herein includes bromo, chloro, iodo and fluoro.

The term purine base includes, but is not limited to, adenine,2-azapurine bases that are optionally substituted imidazo-triazines,imidazo-pyridazines, pyrrolo-pyridazines, pyrrolo-triazines,triazolo-triazines including triazolo[4,5-d]triazines,pyrazolo-triazines including pyrazolo[4,5-d]triazines, N⁶-alkylpurines,N⁶-acylpurines (wherein acyl is C(O)(alkyl, aryl, alkylaryl, orarylalkyl), N⁶-benzylpurine, N⁶-halopurine, N⁶-vinylpurine,N⁶-acetylenic purine, N⁶-acyl purine, N⁶-hydroxyalkyl purine,N⁶-thioalkyl purine, N²-alkylpurines, N²-alkyl-6-thiopurines,C⁵-hydroxyalkyl purine, N²-alkylpurines, N²-alkyl-6-thiopurines,triazolopyridinyl, imidazolopyridinyl, pyrrolopyrimidinyl, andpyrazolopyrimidinyl.

The Base maybe selected from the group consisting of:

The term “acyl” refers to a carboxylic acid ester in which thenon-carbonyl moiety of the ester group is selected from straight,branched, or cyclic alkyl or lower alkyl; alkoxyalkyl includingmethoxymethyl; aralkyl including benzyl; aryloxyalkyl such asphenoxymethyl; aryl including phenyl optionally substituted withhalogen, C₁-C₆ alkyl or C₁-C₆ alkoxy; sulfonate esters such as alkyl oraralkyl sulphonyl including methanesulfonyl; the mono-, di- ortriphosphate ester; trityl or monomethoxytrityl; substituted benzyl;trialkylsilyl as, for eg., dimethyl-t-butylsilyl or diphenylmethylsilyl.Aryl groups in the esters optimally comprise a phenyl group. The term“lower acyl” refers to an acyl group in which the non-carbonyl moiety islower alkyl.

As used herein, the terms “substantially free of” and “substantially inthe absence of” refer to a nucleoside composition that includes at least85-90% by weight, preferably 95%-98% by weight, and even more preferably99%-100% by weight, of the designated enantiomer of that nucleoside. Ina preferred embodiment, the compounds listed in the methods andcompounds of this invention are substantially free of enantiomers otherthan for the one designated.

Similarly, the term “isolated” refers to a nucleoside composition thatincludes at least 85%-90% by weight, preferably 95%-98% by weight, andeven more preferably 99%-100% by weight, of the nucleoside, theremainder comprising other chemical species or enantiomers.

The term “independently” is used herein to indicate that a variable isapplied in any one instance without regard to the presence or absence ofa variable having that same or a different definition within the samecompound. Thus, in a compound in which R″ appears twice and is definedas “independently carbon or nitrogen”, both R″s can be carbon, both R″scan be nitrogen, or one R″ can be carbon and the other nitrogen.

The term “host”, as used herein, refers to a unicellular ormulticellular organism in which the virus can replicate, including celllines and animals, and preferably a human. Alternatively, the host canbe carrying a part of the flavivirus or pestivirus genome, whosereplication or function can be altered by the compounds of the presentinvention. The term host specifically refers to infected cells, cellstransfected with all or part of the flavivirus or pestivirus genome andanimals, in particular, primates (including chimpanzees) and humans. Inmost animal applications of the present invention, the host is a humanpatient. Veterinary applications, in certain indications, however, areclearly anticipated by the present invention such as in chimpanzees.

The term “pharmaceutically acceptable salt or prodrug” is usedthroughout the specification to describe any pharmaceutically acceptableform (ester, phosphate ester, salt of an ester or a related group) of anucleoside compound, which, upon administration to a patient, providesthe nucleoside compound. Pharmaceutically acceptable salts include thosederived from pharmaceutically acceptable inorganic or organic bases andacids. Suitable salts include those derived from alkali metals such aspotassium and sodium, alkaline earth metals such as calcium andmagnesium, among numerous other acids well known in the pharmaceuticalart. Pharmaceutically acceptable prodrugs refer to a compound that ismetabolized, for example, hydrolyzed or oxidized, in the host to formthe compound of the present invention. Typical examples of prodrugsinclude compounds that have biologically labile protecting groups on afunctional moiety of the active compound. Prodrugs include compoundsthat can be oxidized, reduced, aminated, deaminated, hydroxylated,dehydroxylated, hydrolyzed, dehydrolyzed, alkylated, dealkylated,acylated, deacylated, phosphorylated, dephosphorylated to produce theactive compound. The compounds of this invention possess antiviralactivity against flavivirus, pestivirus or HCV, or are metabolized to acompound that exhibits such activity.

Nucleoside Prodrug Formulations

Any of the nucleosides described herein can be administered as anucleotide prodrug to increase the activity, bioavailability, stabilityor otherwise alter the properties of the nucleoside. A number ofnucleotide prodrug ligands are known. In general, alkylation, acylationor other lipophilic modification of the mono-, di- or triphosphate ofthe nucleoside reduces polarity and allows passage into cells. Examplesof substituent groups that can replace one or more hydrogens on thephosphate moiety are alkyl, aryl, steroids, carbohydrates, includingsugars, 1,2-diacylglycerol, alcohols, acyl (including lower acyl); alkyl(including lower alkyl); sulfonate ester including alkyl or arylalkylsulfonyl including methanesulfonyl and benzyl, wherein the phenyl groupis optionally substituted with one or more substituents as provided inthe definition of an aryl given herein; optionally substitutedarylsulfonyl; a lipid, including a phospholipid; an amino acid residueor derivative; a carbohydrate; a peptide; cholesterol; or otherpharmaceutically acceptable leaving group which, when administered invivo, provides a compound wherein R¹ is independently H or phosphate.Many more are described in R. Jones and N. Bischoferger, AntiviralResearch, 1995, 27:1-17. Any of these can be used in combination withthe disclosed nucleosides to achieve a desired effect.

In cases where compounds are sufficiently basic or acidic to form stablenontoxic acid or base salts, administration of the compound as apharmaceutically acceptable salt may be appropriate. Examples ofpharmaceutically acceptable salts are organic acid addition salts formedwith acids, which form a physiological acceptable anion, for example,tosylate, methanesulfonate, acetate, citrate, malonate, tartarate,succinate, benzoate, ascorbate, α-ketoglutarate, and α-glycerophosphate.Suitable inorganic salts may also be formed, including, sulfate,nitrate, bicarbonate, and carbonate salts.

Pharmaceutically acceptable salts may be obtained using standardprocedures well known in the art, for example by reacting a sufficientlybasic compound such as an amine with a suitable acid affording aphysiologically acceptable anion. Alkali metal (for example, sodium,potassium or lithium) or alkaline earth metal (for example calcium)salts of carboxylic acids can also be made.

The active nucleoside can also be provided as a 5′-phosphoether lipid ora 5′-ether lipid, as disclosed in the following references, which areincorporated by reference herein: Kucera, L. S., N. Iyer, E. Leake, A.Raen, Modest E. K., D. L. W., and C. Piantadosi. 1990. “Novelmembrane-interactive ether lipid analogs that inhibit infectious HIV-1production and induce defective virus formation.” AIDS Res. Hum. RetroViruses. 6:491-501; Piantadosi, C., J. Marasco C. J., S. L.Morris-Natschke, K. L. Meyer, F. Gumus, J. R. Surles, K. S. Ishaq, L. S.Kucera, N. Iyer, C. A. Wallen, S. Piantadosi, and E. J. Modest. 1991.

Nonlimiting examples of U.S. patents that disclose suitable lipophilicsubstituents that can be covalently incorporated into the nucleoside,preferably at the 5′-OH position of the nucleoside or lipophilicpreparations, include U.S. Pat. No. 5,149,794 (Sep. 22, 1992, Yatvin etal.); U.S. Pat. No. 5,194,654 (Mar. 16, 1993, Hostetler et al., and U.S.Pat. No. 5,223,263 (Jun. 29, 1993, Hostetler et al.); all of which areincorporated herein by reference. Foreign patent applications thatdisclose lipophilic substituents that can be attached to the nucleosidesof the present invention, or lipophilic preparations, include WO89/02733, WO 90/00555, WO 91/16920, WO 91/18914, WO 93/00910, WO94/26273, WO 96/15132, EP 0 350 287, EP 93917054.4, and WO 91/19721.

Combination and Alternation Therapy

It has been recognized that drug-resistant variants of HCV can emergeafter prolonged treatment with an antiviral agent. Drug resistance mosttypically occurs by mutation of a gene that encodes for an enzyme usedin viral replication. The efficacy of a drug against HCV infection canbe prolonged, augmented, or restored by administering the compound incombination or alternation with a second, and perhaps third, antiviralcompound that induces a different mutation from that caused by theprinciple drug. Alternatively, the pharmacokinetics, biodistriution orother parameter of the drug can be altered by such combination oralternation therapy. In general, combination therapy is typicallypreferred over alternation therapy because it induces multiplesimultaneous stresses on the virus.

Any of the HCV treatments described in the Background of the Inventioncan be used in combination or alternation with the compounds describedin this specification. Nonlimiting examples include:

(1) Interferon

Interferons (IFNs) are compounds that have been commercially availablefor the treatment of chronic hepatitis for nearly a decade. IFNs areglycoproteins produced by immune cells in response to viral infection.IFNs inhibit viral replication of many viruses, including HCV, and whenused as the sole treatment for hepatitis C infection, IFN suppressesserum HCV-RNA to undetectable levels. Additionally, IFN normalizes serumamino transferase levels. Unfortunately, the effects of IFN aretemporary and a sustained response occurs in only 8%-9% of patientschronically infected with HCV (Gary L. Davis. Gastroenterology118:S104-S114, 2000).

A number of patents disclose HCV treatments using interferon-basedtherapies. For example, U.S. Pat. No. 5,980,884 to Blatt et al.discloses methods for re-treatment of patients afflicted with HCV usingconsensus interferon. U.S. Pat. No. 5,942,223 to Bazer et al. disclosesan anti-HCV therapy using ovine or bovine interferon-tau. U.S. Pat. No.5,928,636 to Alber et al. discloses the combination therapy ofinterleukin-12 and interferon alpha for the treatment of infectiousdiseases including HCV. U.S. Pat. No. 5,908,621 to Glue et al. disclosesthe use of polyethylene glycol modified interferon for the treatment ofHCV. U.S. Pat. No. 5,849,696 to Chretien et al. discloses the use ofthymosins, alone or in combination with interferon, for treating HCV.U.S. Pat. No. 5,830,455 to Valtuena et al. discloses a combination HCVtherapy employing interferon and a free radical scavenger. U.S. Pat. No.5,738,845 to Imakawa discloses the use of human interferon tau proteinsfor treating HCV. Other interferon-based treatments for HCV aredisclosed in U.S. Pat. No. 5,676,942 to Testa et al., U.S. Pat. No.5,372,808 to Blatt et al., and U.S. Pat. No. 5,849,696.

(2) Ribavirin (Battaglia, A. M. et al., Ann. Pharmacother, 2000, 34,487-494); Berenguer, M. et al. Antivir. Ther., 1998, 3 (Suppl. 3),125-136).

Ribavirin (1-β-D-ribofuranosyl-1-1,2,4-triazole-3-carboxamide) is asynthetic, non-interferon-inducing, broad spectrum antiviral nucleosideanalog. It is sold under the trade names Virazole™ (The Merck Index,11th edition, Editor: Budavari, S., Merck & Co., Inc., Rahway, N.J.,p1304, 1989); Rebetol (Schering Plough) and Co-Pegasus (Roche). U.S.Pat. No. 3,798,209 and RE29,835 (ICN Pharmaceuticals) disclose and claimribavirin. Ribavirin is structurally similar to guanosine, and has invitro activity against several DNA and RNA viruses includingFlaviviridae (Gary L. Davis. Gastroenterology 118:S104-S114, 2000). U.S.Pat. No. 4,211,771 (to ICN Pharmaceuticals) discloses the use ofribavirin as an antiviral agent. Ribavirin reduces serum aminotransferase levels to normal in 40% of patients, but it does not lowerserum levels of HCV-RNA (Gary L. Davis. Gastroenterology 118:S104-S114,2000). Thus, ribavirin alone is not effective in reducing viral RNAlevels. Additionally, ribavirin has significant toxicity and is known toinduce anemia.

Combination of Interferon and Ribavirin

Schering-Plough sells ribavirin as Rebetol® capsules (200 mg) foradministration to patients with HCV. The U.S. FDA has approved Rebetolcapsules to treat chronic HCV infection in combination with Schering'salpha interferon-2b products Intron® A and PEG-Intron™. Rebetol capsulesare not approved for monotherapy (i.e., administration independent ofIntron®A or PEG-Intron), although Intron A and PEG-Intron are approvedfor monotherapy (i.e., administration without ribavirin). Hoffman LaRoche is selling ribavirin under the name Co-Pegasus in Europe and theUnited States, also for use in combination with interferon for thetreatment of HCV. Other alpha interferon products include Roferon-A(Hoffmann-La Roche), Infergen® (Intermune, formerly Amgen's product),and Weliferon® (Wellcome Foundation) are currently FDA-approved for HCVmonotherapy. Interferon products currently in development for HCVinclude: Roferon-A (interferon alfa-2a) by Roche, PEGASYS (pegylatedinterferon alfa-2a) by Roche, INFERGEN (interferon alfacon-1) byInterMune, OMNIFERON (natural interferon) by Viragen, ALBUFERON by HumanGenome Sciences, REBIF (interferon beta-1a) by Ares-Serono, OmegaInterferon by BioMedicine, Oral Interferon Alpha by AmarilloBiosciences, and Interferon gamma-1b by InterMune.

The combination of IFN and ribavirin for the treatment of HCV infectionhas been reported to be effective in the treatment of IFN naïve patients(for example, Battaglia, A. M. et al., Ann. Pharmacother. 34:487-494,2000). Combination treatment is effective both before hepatitis developsand when histological disease is present (for example, Berenguer, M. etal. Antivir. Ther. 3(Suppl. 3):125-136, 1998). Currently, the mosteffective therapy for HCV is combination therapy of pegylated interferonwith ribavirin (2002 NIH Consensus Development Conference on theManagement of Hepatitis C). However, the side effects of combinationtherapy can be significant and include hemolysis, flu-like symptoms,anemia, and fatigue (Gary L. Davis. Gastroenterology 118:S104-S114,2000).

(3) Protease inhibitors have been developed for the treatment ofFlaviviridae infections. Examples, include, but are not limited to thefollowing

Substrate-based NS3 protease inhibitors (see, for example, Attwood etal., Antiviral peptide derivatives, PCT WO 98/22496, 1998; Attwood etal., Antiviral Chemistry and Chemotherapy 1999, 10, 259-273; Attwood etal., Preparation and use of amino acid derivatives as anti-viral agents,German Patent Pub. DE 19914474; Tung et al. Inhibitors of serineproteases, particularly hepatitis C virus NS3 protease, PCT WO98/17679), including alphaketoamides and hydrazinoureas, and inhibitorsthat terminate in an electrophile such as a boronic acid or phosphonate(see, for example, Llinas-Brunet et al, Hepatitis C inhibitor peptideanalogues, PCT WO 99/07734);

Non-substrate-based inhibitors such as2,4,6-trihydroxy-3-nitro-benzamide derivatives (see, for example, SudoK. et al., Biochemical and Biophysical Research Communications, 1997,238, 643-647; Sudo K. et al. Antiviral Chemistry and Chemotherapy, 1998,9, 186), including RD3-4082 and RD3-4078, the former substituted on theamide with a 14 carbon chain and the latter processing apara-phenoxyphenyl group;

Phenanthrenequinones possessing activity against protease, for examplein a SDS-PAGE and/or autoradiography assay, such as, for example, Sch68631, isolated from the fermentation culture broth of Streptomyces sp.,(see, for example, Chu M. et al., Tetrahedron Letters, 1996, 37,7229-7232), and Sch 351633, isolated from the fungus Penicilliumgriseofulvum, which demonstrates activity in a scintillation proximityassay (see, for example, Chu M. et al., Bioorganic and MedicinalChemistry Letters 9, 1949-1952); and

Selective NS3 inhibitors, for example, based on the macromolecule elginc, isolated from leech (see, for example, Qasim M. A. et al.,Biochemistry, 1997, 36, 1598-1607). Nanomolar potency against the HCVNS3 protease enzyme has been achieved by the design of selectiveinhibitors based on the macromolecule eglin c. Eglin c, isolated fromleech, is a potent inhibitor of several serine proteases such as S.griseus proteases A and B, α-chymotrypsin, chymase and subtilisin.

Several U.S. patents disclose protease inhibitors for the treatment ofHCV. Non-limiting examples include, but are not limited to thefollowing. U.S. Pat. No. 6,004,933 to Spruce et al. discloses a class ofcysteine protease inhibitors for inhibiting HCV endopeptidase. U.S. Pat.No. 5,990,276 to Zhang et al. discloses synthetic inhibitors ofhepatitis C virus NS3 protease. The inhibitor is a subsequence of asubstrate of the NS3 protease or a substrate of the NS4A cofactor. Theuse of restriction enzymes to treat HCV is disclosed in U.S. Pat. No.5,538,865 to Reyes et al. Peptides as NS3 serine protease inhibitors ofHCV are disclosed in WO 02/008251 to Corvas International, Inc, and WO02/08187 and WO 02/008256 to Schering Corporation. HCV inhibitortripeptides are disclosed in U.S. Pat. Nos. 6,534,523, 6,410,531, and6,420,380 to Boehringer Ingelheim and WO 02/060926 to Bristol MyersSquibb. Diaryl peptides as NS3 serine protease inhibitors of HCV aredisclosed in WO 02/48172 to Schering Corporation. Iridazoleidinones asNS3 serine protease inhibitors of HCV are disclosed in WO 02/08198 toSchering Corporation and WO 02/48157 to Bristol Myers Squibb. WO98/17679 to Vertex Pharmaceuticals and WO 02/48116 to Bristol MyersSquibb also disclose HCV protease inhibitors.

(4) Thiazolidine derivatives, for example, that show relevant inhibitionin a reverse-phase HPLC assay with an NS3/4A fusion protein and NS5A/5Bsubstrate (see, for example, Sudo K. et al., Antiviral Research, 1996,32, 9-18), especially compound RD-1-6250, possessing a fused cinnamoylmoiety substituted with a long alkyl chain, RD4 6205 and RD4 6193;

(5) Thiazolidines and benzanilides, for example, as identified inKakiuchi N. et al. J. EBS Letters 421, 217-220; Takeshita N. et al.Analytical Biochemistry, 1997, 247, 242-246;

(6) Helicase inhibitors (see, for example, Diana G. D. et al.,Compounds, compositions and methods for treatment of hepatitis C, U.S.Pat. No. 5,633,358; Diana G. D. et al., Piperidine derivatives,pharmaceutical compositions thereof and their use in the treatment ofhepatitis C, PCT WO 97/36554);

(7) Polymerase inhibitors such as

-   -   i) nucleotide analogues, such as gliotoxin (see, for example,        Ferrari R. et al. Journal of Virology, 1999, 73, 1649-1654);    -   ii) the natural product cerulenin (see, for example, Lohmann V.        et al., Virology, 1998, 249, 108-118); and    -   iii) non-nucleoside polymerase inhibitors, including, for        example, compound R803 (see, for example, WO 04/018463 A2 and WO        03/040112 A1, both to Rigel Pharmaceuticals, Inc.); substituted        diamine pyrimidines (see, for example, WO 03/063794 A2 to Rigel        Pharmaceuticals, Inc.); benzimidazole derivatives (see, for        example, Bioorg. Med. Chem. Lett., 2004, 14:119-124 and Bioorg.        Med. Chem. Lett., 2004, 14:967-971, both to Boehringer Ingelheim        Corporation); N,N-disubstituted phenylalanines (see, for        example, J. Biol. Chem., 2003, 278:9495-98 and J. Med. Chem.,        2003, 13:1283-85, both to Shire Biochem, Inc.); substituted        thiophene-2-carboxylic acids (see, for example, Bioorg. Med.        Chem. Lett., 2004, 14:793-796 and Bioorg. Med. Chem. Lett.,        2004, 14:797-800, both to Shire Biochem, Inc.); α,γ-diketoacids        (see, for example, J. Med. 5 Chem., 2004, 14-17 and WO 00/006529        A1, both to Merck & Co., Inc.); and meconic acid derivatives        (see, for example, Bioorg. Med. Chem. Lett., 2004, 3257-3261, WO        02/006246 A1 and WO03/062211 A1, all to IRBM Merck & Co., Inc.);

(8) Antisense phosphorothioate oligodeoxynucleotides (S—ODN)complementary, for example, to sequence stretches in the 5′ non-codingregion (NCR) of the virus (see, for example, Alt M. et al., Hepatology,1995, 22, 707-717), or to nucleotides 326-348 comprising the 3′ end ofthe NCR and nucleotides 371-388 located in the core coding region of theHCV RNA (see, for example, Alt M. et al., Archives of Virology, 1997,142, 589-599; Galderisi U. et al., Journal of Cellular Physiology, 1999,181, 251-257).

(9) Inhibitors of IRES-dependent translation (see, for example, Ikeda Net al., Agent for the prevention and treatment of hepatitis C, JapanesePatent Pub. JP-08268890; Kai Y. et al. Prevention and treatment of viraldiseases, Japanese Patent Pub. JP-10101591).

(10) Nuclease-resistant ribozymes (see, for example, Maccjak, D. J. etal., Hepatology 1999, 30, abstract 995; U.S. Pat. No. 6,043,077 toBarber et al., and U.S. Pat. Nos. 5,869,253 and 5,610,054 to Draper etal.).

(11) Nucleoside analogs have also been developed for the treatment ofFlaviviridae infections.

Idenix Pharmaceuticals, Ltd. discloses branched nucleosides, and theiruse in the treatment of HCV and flaviviruses and pestiviruses in U.S.patent Publication Nos. 2003/0050229 A1, 2004/0097461 A1, 2004/0101535A1, 2003/0060400 A1, 2004/0102414 A1, 2004/0097462 A1, and 2004/0063622A1 which correspond to International Publication Nos. WO 01/90121 and WO01/92282. A method for the treatment of hepatitis C infection (andflaviviruses and pestiviruses) in humans and other host animals isdisclosed in the Idenix publications that includes administering aneffective amount of a biologically active 1′, 2′, 3′ or 4′-branched β-Dor β-L nucleosides or a pharmaceutically acceptable salt or prodrugthereof, administered either alone or in combination, optionally in apharmaceutically acceptable carrier. See also U.S. patent PublicationNos. 2004/0006002 and 2004/0006007 as well as WO 03/026589 and WO03/026675. Idenix Pharmaceuticals, Ltd. also discloses in U.S. patentPublication No. 2004/0077587 pharmaceutically acceptable branchednucleoside prodrugs, and their use in the treatment of HCV andflaviviruses and pestiviruses in prodrugs. See also PCT Publication Nos.WO 04/002422, WO 04/002999, and WO 04/003000. Further, IdenixPharmaceuticals, Ltd. also discloses in WO 04/046331 Flaviviridaemutations caused by biologically active 2′-branched β-D or β-Lnucleosides or a pharmaceutically acceptable salt or prodrug thereof.

Biota Inc. discloses various phosphate derivatives of nucleosides,including 1′, 2′, 3′ or 4′-branched β-D or β-L nucleosides, for thetreatment of hepatitis C infection in International Patent PublicationWO 03/072757.

Emory University and the University of Georgia Research Foundation, Inc.(UGARF) discloses the use of 2′-fluoronucleosides for the treatment ofHCV in U.S. Pat. No. 6,348,587. See also U.S. patent Publication No.2002/0198171 and International Patent Publication WO 99/43691.

BioChem Pharma Inc. (now Shire Biochem, Inc.) discloses the use ofvarious 1,3-dioxolane nucleosides for the treatment of a Flaviviridaeinfection in U.S. Pat. No. 6,566,365. See also U.S. Pat. Nos. 6,340,690and 6,605,614; U.S. patent Publication Nos. 2002/0099072 and2003/0225037, as well as International Publication No. WO 01/32153 andWO 00/50424.

BioChem Pharma Inc. (now Shire Biochem, Inc.) also discloses variousother 2′-halo, 2′-hydroxy and 2′-alkoxy nucleosides for the treatment ofa Flaviviridae infection in U.S. patent Publication No. 2002/0019363 aswell as International Publication No. WO 01/60315 (PCT/CA01/00197; filedFeb. 19, 2001).

ICN Pharmaceuticals, Inc. discloses various nucleoside analogs that areuseful in modulating immune response in U.S. Pat. Nos. 6,495,677 and6,573,248. See also WO 98/16184, WO 01/68663, and WO 02/03997.

U.S. Pat. No. 6,660,721; U.S. patent Publication Nos. 2003/083307 A1,2003/008841 A1, and 2004/0110718; as well as International PatentPublication Nos. WO 02/18404; WO 02/100415, WO 02/094289, and WO04/043159; filed by F. Hoffmann-La Roche AG, discloses variousnucleoside analogs for the treatment of HCV RNA replication.

Pharmasset Limited discloses various nucleosides and antimetabolites forthe treatment of a variety of viruses, including Flaviviridae, and inparticular HCV, in U.S. patent Publication Nos. 2003/0087873,2004/0067877, 2004/0082574, 2004/0067877, 2004/002479, 2003/0225029, and2002/00555483, as well as International Patent Publication Nos. WO02/32920, WO 01/79246, WO 02/48165, WO 03/068162, WO 03/068164 and WO2004/013298.

Merck & Co., Inc. and Isis Pharmaceuticals disclose in U.S. patentPublication Nos. 2002/0147160, 2004/0072788, 2004/0067901, and2004/0110717; as well as the corresponding International PatentPublication Nos. WO 02/057425 (PCT/US02/01531; filed Jan. 18, 2002) andWO 02/057287 (PCT/US02/03086; filed Jan. 18, 2002) various nucleosides,and in particular several pyrrolopyrimidine nucleosides, for thetreatment of viruses whose replication is dependent upon RNA-dependentRNA polymerase, including Flaviviridae, and in particular HCV. See alsoWO 2004/000858, WO 2004/003138, WO 2004/007512, and WO 2004/009020.

U.S. patent Publication No. 2003/028013 A1 as well as InternationalPatent Publication Nos. WO 03/051899, WO 03/061576, WO 03/062255 WO03/062256, WO 03/062257, and WO 03/061385, filed by Ribapharm, also aredirected to the use of certain nucleoside analogs to treat hepatitis Cvirus.

Genelabs Technologies disclose in U.S. patent Publication No.2004/0063658 as well as International Patent Publication Nos. WO03/093290 and WO 04/028481 various base modified derivatives ofnucleosides, including 1′, 2′, 35′ or 4′-branched β-D or β-Lnucleosides, for the treatment of hepatitis C infection.

Eldrup et al. (Oral Session V, Hepatitis C Virus, Flaviviridae; 16^(th)International Conference on Antiviral Research (Apr. 27, 2003, Savannah,Ga.) p. A75) described the structure activity relationship of2′-modified nucleosides for inhibition of HCV.

Bhat et al (Oral Session V, Hepatitis C Virus, Flaviviridae; 16^(th)International Conference on Antiviral Research (Apr. 27, 2003, Savannah,Ga.); p A75) describe the synthesis and pharmacokinetic properties ofnucleoside analogues as possible inhibitors of HCV RNA replication. Theauthors report that 2′-modified nucleosides demonstrate potentinhibitory activity in cell-based replicon assays.

Olsen et al. (Oral Session V, Hepatitis C Virus, Flaviviridae; 16^(th)International Conference on Antiviral Research (Apr. 27, 2003, Savannah,Ga.) p A76) also described the effects of the 2′-modified nucleosides onHCV RNA replication.

(12) Other miscellaneous compounds including 1-amino-alkylcyclohexanes(for example, U.S. Pat. No. 6,034,134 to Gold et al.), alkyl lipids (forexample, U.S. Pat. No. 5,922,757 to Chojkier et al.), vitamin E andother antioxidants (for example, U.S. Pat. No. 5,922,757 to Chojkier etal.), squalene, amantadine, bile acids (for example, U.S. Pat. No.5,846,964 to Ozeki et al.), N-(phosphonoacetyl)-L-aspartic acid (forexample, U.S. Pat. No. 5,830,905 to Diana et al.), benzenedicarboxamides(for example, U.S. Pat. No. 5,633,388 to Diana et al.), polyadenylicacid derivatives (for example, U.S. Pat. No. 5,496,546 to Wang et al.),2′,3′-dideoxyinosine (for example, U.S. Pat. No. 5,026,687 to Yarchoanet al.), benzimidazoles (for example, U.S. Pat. No. 5,891,874 toColacino et al.), plant extracts (for example, U.S. Pat. No. 5,837,257to Tsai et al., U.S. Pat. No. 5,725,859 to Omer et al., and U.S. Pat.No. 6,056,961), and piperidenes (for example, U.S. Pat. No. 5,830,905 toDiana et al.).

(13) Other compounds currently in clinical development for treatment ofhepatitis C virus include, for example: Interleukin-10 bySchering-Plough, IP-501 by Interneuron, Merimebodib VX-497 by Vertex,AMANTADINE® (Symmetrel) by Endo Labs Solvay, HEPTAZYME® by RPI, IDN-6556by Idun Pharma., XTL-002 by XTL., HCV/MF59 by Chiron, CIVACIR®(Hepatitis C Immune Globulin) by NABI, LEVOVIRIN® by ICN/Ribapharm,VIRAMIDINE® by ICN/Ribapharm, ZADAXIN® (thymosin alfa-1) by Sci Clone,thymosin plus pegylated interferon by Sci Clone, CEPLENE®) (histaminedihydrochloride) by Maxim, VX 950/LY 570310 by Vertex/Eli Lilly, ISIS14803 by Isis Pharmaceutical/Elan, IDN-6556 by Idun Pharmaceuticals,Inc., JTK 003 by AKROS Pharma, BILN-2061 by Boehringer Ingelheim,CellCept (mycophenolate mofetil) by Roche, T67, a β-tubulin inhibitor,by Tularik, a therapeutic vaccine directed to E2 by Innogenetics, FK788by Fujisawa Healthcare, Inc., IdB 1016 (Siliphos, oralsilybin-phosphatdylcholine phytosome), RNA replication inhibitors(VP50406) by ViroPharma/Wyeth, therapeutic vaccine by Intercell,therapeutic vaccine by Epimmune/Genencor, IRES inhibitor by Anadys, ANA245 and ANA 246 by Anadys, immunotherapy (Therapore) by Avant, proteaseinhibitor by Corvas/SChering, helicase inhibitor by Vertex, fusioninhibitor by Trimeris, T cell therapy by CellExSys, polymerase inhibitorby Biocryst, targeted RNA chemistry by PTC Therapeutics, Dication byImmtech, Int., protease inhibitor by Agouron, protease inhibitor byChiron/Medivir, antisense therapy by AVI BioPharma, antisense therapy byHybridon, hemopurifier by Aethlon Medical, therapeutic vaccine by Merix,protease inhibitor by Bristol-Myers Squibb/Axys, Chron-VacC, atherapeutic vaccine, by Tripep, UT 231B by United Therapeutics,protease, helicase and polymerase inhibitors by Genelabs Technologies,IRES inhibitors by Immusol, R803 by Rigel Pharmaceuticals, INFERGEN®(interferon alphacon-1) by InterMune, OMNIEFERON® (natural interferon)by Viragen, ALBUFERON® by Human Genome Sciences, REBIF® (interferonbeta-1a) by Ares-Serono, Omega Interferon by BioMedicine, OralInterferon Alpha by Amarillo Biosciences, interferon gamma, interferontau, and Interferon gamma-1b by InterMune.

Pharmaceutical Compositions

Hosts, including humans, infected with pestivirus, flavivirus, HCV oranother organism replicating through a RNA-dependent RNA viralpolymerase, can be treated by administering to the patient an effectiveamount of the active compound or a pharmaceutically acceptable prodrugor salt thereof in the presence of a pharmaceutically acceptable carrieror diluent. The active materials can be administered by any appropriateroute, for example, orally, parenterally, intravenously, intradermally,subcutaneously, or topically, in liquid or solid form.

A preferred dose of the compound for pestivirus, flavivirus or HCV willbe in the range from about 1 to 50 mg/kg, preferably 1 to 20 mg/kg, ofbody weight per day, more generally 0.1 to about 100 mg per kilogrambody weight of the recipient per day. The effective dosage range of thepharmaceutically acceptable salts and prodrugs can be calculated basedon the weight of the parent nucleoside to e delivered. If the salt orprodrug exhibits activity in itself, the effective dosage can beestimated as above using the weight of the salt or prodrug, or by othermeans known to those skilled in the art.

The compound is conveniently administered in unit any suitable dosageform, including but not limited to one containing 7 to 3000 mg, or 70 to1400 mg of active ingredient per unit dosage form. An oral dosage in oneembodiment is 50-1000 mg. In another embodiment, the dosage formcontains 0.5-500 mg; or 0.5-100 mg; or 0.5-50 mg; or 0.5-25 mg; or1.0-10 mg.

Ideally the active ingredient should be administered to achieve peakplasma concentrations of the active compound of from about 0.2 to 70 μM,preferably about 1.0 to 10 μM. This may be achieved, for example, by theintravenous injection of a 0.1 to 5% solution of the active ingredient,optionally in saline, or administered as a bolus of the activeingredient.

The concentration of active compound in the drug composition will dependon absorption, inactivation and excretion rates of the drug as well asother factors known to those of skill in the art. It is to be noted thatdosage values will also vary with the severity of the condition to bealleviated. It is to be further understood that for any particularsubject, specific dosage regimens should be adjusted over time accordingto the individual need and the professional judgment of the personadministering or supervising the administration of the compositions, andthat the concentration ranges set forth herein are exemplary only andare not intended to limit the scope or practice of the claimedcomposition. The active ingredient may be administered at once, or maybe divided into a number of smaller doses to be administered at varyingintervals of time.

A preferred mode of administration of the active compound is oral. Oralcompositions will generally include an inert diluent or an ediblecarrier. They may be enclosed in gelatin capsules or compressed intotablets. For the purpose of oral therapeutic administration, the activecompound can be incorporated with excipients and used in the form oftablets, troches, or capsules. Pharmaceutically compatible bindingagents, and/or adjuvant materials can e included as part of thecomposition.

The tablets, pills, capsules, troches and the like can contain any ofthe following ingredients, or compounds of a similar nature: a bindersuch as microcrystalline cellulose, gum tragacanth or gelatin; anexcipient such as starch or lactose, a disintegrating agent such asalginic acid, Primogel, or corn starch; a lubricant such as magnesiumstearate or Sterotes; a glidant such as colloidal silicon dioxide; asweetening agent such as sucrose or saccharin; or a flavoring agent suchas peppermint, methyl salicylate, or orange flavoring. When the dosageunit form is a capsule, it can contain, in addition to material of theabove type, a liquid carrier such as a fatty oil. In addition, dosageunit forms can contain various other materials which modify the physicalform of the dosage unit, for example, coatings of sugar, shellac, orother enteric agents.

The compound can be administered as a component of an elixir,suspension, syrup, wafer, chewing gum or the like. A syrup may contain,in addition to the active compounds, sucrose as a sweetening agent andcertain preservatives, dyes and colorings and flavors.

The compound or a pharmaceutically acceptable prodrug or salts thereofcan also be mixed with other active materials that do not impair thedesired action, or with materials that supplement the desired action,such as antibiotics, antifungals, anti-inflammatories, or otherantivirals, including other nucleoside compounds. Solutions orsuspensions used for parenteral, intradermal, sucutaneous, or topicalapplication can include the following components: a sterile diluent suchas water for injection, saline solution, fixed oils, polyethyleneglycols, glycerine, propylene glycol or other synthetic solvents;antibacterial agents such as benzyl alcohol or methyl parabens;antioxidants such as ascorbic acid or sodium bisulfite; chelating agentssuch as ethylenediaminetetraacetic acid; buffers such as acetates,citrates or phosphates and agents for the adjustment of tonicity such assodium chloride or dextrose. The parental preparation can be enclosed inampoules, disposable syringes or multiple dose vials made of glass orplastic.

If administered intravenously, preferred carriers are physiologicalsaline or phosphate buffered saline (PBS).

In a preferred embodiment, the active compounds are prepared withcarriers that will protect the compound against rapid elimination fromthe body, such as a controlled release formulation, including implantsand microencapsulated delivery systems. biodegradable, biocompatiblepolymers can be used, such as ethylene vinyl acetate, polyanhydrides,polyglycolic acid, collagen, polyorthoesters and polylactic acid.Methods for preparation of such formulations will be apparent to thoseskilled in the art. The materials can also be obtained commercially fromAlza Corporation.

Liposomal suspensions (including liposomes targeted to infected cellswith monoclonal antibodies to viral antigens) are also preferred aspharmaceutically acceptable carriers. These may be prepared according tomethods known to those skilled in the art, for example, as described inU.S. Pat. No. 4,522,811 (which is incorporated herein by reference inits entirety). For example, liposome formulations may be prepared bydissolving appropriate lipid(s) (such as stearoyl phosphatidylethanolamine, stearoyl phosphatidyl choline, arachadoyl phosphatidylcholine, and cholesterol) in an inorganic solvent that is thenevaporated, leaving behind a thin film of dried lipid on the surface ofthe container. An aqueous solution of the active compound or itsmonophosphate, diphosphate, and/or triphosphate derivatives is thenintroduced into the container. The container is then swirled by hand tofree lipid material from the sides of the container and to disperselipid aggregates, thereby forming the liposomal suspension.

Processes for the Preparation of Active Compounds

The nucleosides of the present invention can be synthesized by any meansknown in the art. In particular, the synthesis of the presentnucleosides can be achieved by either alkylating the appropriatelymodified sugar, followed by glycosylation or glycosylation followed byalkylation of the nucleoside, though preferably alkylating theappropriately modified sugar, followed by glycosylation. The followingnon-limiting embodiments illustrate some general methodology to obtainthe nucleosides of the present invention.

A. General Synthesis of 1′-C-Branched Nucleosides

1′-C branched ribonucleosides of the following structures:

wherein

-   -   R is H, phosphate (including mono-, di-, or triphosphate or a        stabilized phosphate prodrug) or phosphonate;    -   n is 0-2;    -   when X is CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,        C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,        CH—S-alkenyl, CH—S-alkynyl, CH-halogen, or C-(halogen)₂,    -   then each R¹ and R^(1′) is independently H, OH, optionally        substituted alkyl including lower alkyl, azido, cyano,        optionally substituted alkenyl or alkynyl, —C(O)O-(alkyl),        —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),        —O(acyl), —O(lower acyl), —O(alkyl), —O(lower alkyl),        —O(alkenyl), —O(alkynyl), halogen, halogenated alkyl, —NO₂,        —NH₂, —NH(lower alkyl), —N(lower alkyl)₂, —NH(acyl), —N(acyl)₂,        —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, S(O)N-alkyl,        S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, wherein alkyl,        alkenyl, and/or alkynyl maybe optionally substituted;    -   when X is O, S[O]_(n), NH, N-alkyl, N-alkenyl, N-alkynyl,        S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, or SCH-halogen,    -   then each R¹ and R^(1′) is independently H, optionally        substituted alkyl including lower alkyl, azido, cyano,        optionally substituted alkenyl or alkynyl, —C(O)O-(alkyl),        —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),        halogenated alkyl, —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂,        —C(H)═N—NH₂, C(S)NH₂, C(S)NH(alkyl), or C(S)N(alkyl)₂, wherein        alkyl, alkenyl, and/or alkynyl maybe optionally substituted;    -   when X* is CY³;    -   then each R¹ is independently H, OH, optionally substituted        alkyl including lower alkyl, azido, cyano, optionally        substituted alkenyl or alkynyl, —C(O)O-(alkyl), —C(O)O(lower        alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl), —O(acyl), —O(lower        acyl), —O(alkyl), —O(lower alkyl), —O(alkenyl), —O(alkynyl),        halogen, halogenated alkyl, —NO₂, —NH₂, —NH(lower alkyl),        —N(lower alkyl)₂, —NH(acyl), —N(acyl)₂, —C(O)NH₂,        —C(O)NH(alkyl), and —C(O)N(alkyl)₂, wherein an optional        substitution on alkyl, alkenyl, and/or alkynyl may be one or        more halogen, hydroxy, alkoxy or alkylthio groups taken in any        combination; and    -   Y³ is hydrogen, alkyl, bromo, chloro, fluoro, iodo, azido,        cyano, alkenyl, alkynyl, —C(O)O(alkyl), —C(O)O(lower alkyl),        CF₃, —CONH₂, —CONH(alkyl), —CON(alkyl)₂;    -   each R² and R³ independently is OH, NH₂, SH, F, Cl, Br, I, CN,        NO₂, —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, N₃, optionally        substituted alkyl including lower alkyl, optionally substituted        alkenyl or alkynyl, halogenated alkyl, —C(O)O-(alkyl),        —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),        —O(acyl), —O(alkyl), —O(alkenyl), —O(alkynyl), —OC(O)NH₂, NC,        C(O)OH, SCN, OCN, —S(alkyl), —S(alkenyl), —S(alkynyl),        —NH(alkyl), —N(alkyl)₂, —NH(alkenyl), —NH(alkynyl), an amino        acid residue or derivative, a prodrug or leaving group that        provides OH in vivo, or an optionally substituted 3-7 membered        heterocyclic ring having O, S and/or N independently as a        heteroatom taken alone or in combination;    -   each R^(2′) and R^(3′) independently is H; optionally        substituted alkyl, alkenyl, or alkynyl; —C(O)O(alkyl),        —C(O)O(lower alkyl), —C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂,        —C(O)NH(alkyl), —C(O)N(alkyl)₂, —O(acyl), —O(lower acyl),        —O(alkyl), —O(lower alkyl), —O(alkenyl), halogen, halogenated        alkyl and particularly CF₃, azido, cyano, NO₂, —S(alkyl),        —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂,        —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂, and R₃ at        3′-C may also be OH;    -   Base is selected from the group consisting of:        wherein    -   each A independently is N or C—R⁵;    -   W is H, OH, —O(acyl), —O(C₁₋₄ alkyl), —O(alkenyl), —O(alkynyl),        —OC(O)R⁴R⁴, —OC(O)N R⁴R⁴, SH, —S(acyl), —S(C₁₋₄ alkyl), NH₂,        NH(acyl), N(acyl)₂, NH(C₁₋₄ alkyl), N(C₁₋₄ alkyl)₂,        —N(cycloalkyl) C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₃₋₆        cycloalkylamino, halogen, C₁₋₄ alkyl, C₁₋₄ alkoxy, CN, SCN, OCN,        SH, N₃, NO₂, NH═NH₂, N₃, NHOH, —C(O)NH₂, —C(O)NH(acyl),        —C(O)N(acyl)₂, —C(O)NH(C₁₋₄ alkyl), —C(O)N(C₁₋₄ alkyl)₂,        —C(O)N(alkyl)(acyl), or halogenated alkyl;    -   Z is O, S, NH, N—OH, N—NH₂, NH(alkyl), N(alkyl)₂, N-cycloalkyl,        alkoxy, CN, SCN, OCN, SH, NO₂, NH₂, N₃, NH═NH, NH(alkyl),        N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂.    -   each R⁴ is independently H, acyl, or C₁₋₆ alkyl;    -   each R⁵ is independently H, Cl, Br, F, I, CN, OH, optionally        substituted alkyl, alkenyl or alkynyl, carboxy, C(═NH)NH₂, C₁₋₄        alkoxy, C₁₋₄ alkyloxycarbonyl, N₃, NH₂, NH(alkyl), N(alkyl)₂,        NO₂, N₃, halogenated alkyl especially CF₃, C₁₋₄ alkylamino,        di(C₁₋₄ alkyl)amino, C₃₋₆ cycloalkylamino, C₁₋₆ alkoxy, SH,        —S(C₁₋₄ alkyl), —S(C₁₋₄ alkenyl), —S(C₁₋₄ alkynyl), C₁₋₆        alkylthio, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl, C₃₋₆        cycloalkylamino -alkenyl, -alkynyl, —(O)alkyl, —(O)alkenyl,        —(O)alkynyl, —(O)acyl, —O(C₁₋₄ alkyl), —O(C₁₋₄ alkenyl), —O(Cl₄        alkynyl), —O—C(O)NH₂, —OC(O)N(alkyl), —OC(O)R′R″, —C(O)OH,        C(O)O-alkyl, C(O)O-alkenyl, C(O)O-alkynyl, S-alkyl, S-acyl,        S-alkenyl, S-alkynyl, SCN, OCN, NC, —C(O)—NH₂, C(O)NH(alkyl),        C(O)N(alkyl)₂, C(O)NH(acyl), C(O)N(acyl)₂, (S)—NH₂, NH-alkyl,        N(dialkyl)₂, NH-acyl, N-diacyl, or a 3-7 membered heterocycle        having O, S, or N taken independently in any combination;    -   each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,        C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,        C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino,        C₁₋₆ alkoxy, C₁₋₆ alkylsulfonyl, or (C₁₋₄ alkyl)₀₋₂ aminomethyl;        and all tautomeric, enantiomeric and stereoisomeric forms        thereof;    -   with the caveat that when X is S in Formula (I), then the        compound is not        5-(4-amino-imidazo[4,5-d][1,2,3]triazin-7-yl)-2-hydroxymethyl-tetrahydro-thiophen-3-ol        or        7-(4-hydroxy-5-hydroxy-methyl-tetrahydro-thiophen-2-yl)-3,7-dihydro-imidazo[4,5-d][1,2,3]triazin-4-one,        can be prepared according to Schemes 1, 2 or 7 below.        Modification from the Lactone

The key starting material for this process is an appropriatelysubstituted lactone. The lactone may be purchased or can be prepared byany known means including standard epimerization, substitution andcyclization techniques. The lactone optionally can be protected with asuitable protecting group, preferably with an acyl or silyl group, bymethods well known to those skilled in the art, as taught by Greene etal., Protective Groups in Organic Synthesis, John Wiley and Sons, SecondEdition, 1991. The protected lactone can then be coupled with a suitablecoupling agent, such as an organometallic carbon nucleophile like aGrignard reagent, an organolithium, lithium dialkylcopper or R⁶—SiMe₃ inTAF with the appropriate non-protic solvent at a suitable temperature,to give the 1′-alkylated sugar.

The optionally activated sugar can then be coupled to the base bymethods well known to those skilled in the art, as taught by Townsend,Chemistry of Nuceleotides, Plenum Press, 1994. For example, an acylatedsugar can be coupled to a silylated base with a Lewis acid such as tintetrachloride, titanium tetrachloride, or trimethylsilyltriflate in theappropriate solvent at a suitable temperature.

Subsequently, the nucleoside can be deprotected by methods well known tothose skilled in the art, as taught by Greene et al., Protective Groupsin Organic Synthesis, John Wiley and Sons, Second Edition, 1991.

In a particular embodiment, the 1′-C-branched ribonucleoside is desired.The synthesis of a ribonucleoside is shown in Scheme 1. Alternatively,deoxyribonucleoside is desired. To obtain these nucleosides, the formedribonucleoside an optionally be protected by methods well known to thoseskilled in the art, as taught by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991, and thenthe 2′-OH can be reduced with a suitable reducing agent. Optionally, the2′-OH can be activated to facilitate reduction as, for example, via theBarton reduction.

Alternative Method for the Preparation of 1′-C-Branched Nucleosides

The key starting material for this process is an appropriatelysubstituted hexose. The hexose can be purchased or can be prepared byany known means including standard epimerization (as, for example, viaalkaline treatment), substitution and coupling techniques. The hexosecan be protected selectively to give the appropriate hexa-furanose, astaught by Townsend, Chemistry of Nucleosides and Nucleotides, PlenumPress, 1994.

The 1′-OH optionally can be activated to a suitable leaving group suchas an acyl group or a halogen via acylation or halogenation,respectively. The optionally activated sugar can then be coupled to thebase by methods well known to those skilled in the art, as taught byTownsend, Chemistry of Nucleosides and Nucleotides, Plenum Press, 1994.For example, an acylated sugar can be coupled to a silylated base with aLewis acid, such as tin tetrachloride, titanium tetrachloride, ortrimethylsilyltriflate in an appropriate solvent at a suitabletemperature. Alternatively, a halo-sugar can be coupled to a silylatedbase in the presence of trimethylsilyltriflate.

The 1′-CH₂—OH, if protected, selectively can be deprotected by methodswell known in the art. The resultant primary hydroxyl can be reduced togive the methyl, using a suitable reducing agent. Alternatively, thehydroxyl can be activated prior to reduction to facilitate the reaction,i.e., via the Barton reduction. In an alternate embodiment, the primaryhydroxyl can be oxidized to the aldehyde, then coupled with a carbonnucleophile such as a Grignard reagent, an organolithium, lithiumdialkylcopper or R⁶—SiMe₃ in TAF with an appropriate non-protic solventat a suitable temperature.

In a particular embodiment, the 1′-C-branched ribonucleoside is desired.The synthesis of a ribonucleoside is shown in Scheme 2. Alternatively,deoxyribonucleoside is desired. To obtain these nucleosides, the formedribonucleoside optionally can be protected by methods well known tothose skilled in the art, as taught by Greene et al., Protective Groupsin Organic Synthesis, John Wiley and Sons, Second Edition, 1991, andthen the 2′-OH can be reduced with a suitable reducing agent.Optionally, the 2′-OH can be activated to facilitate reduction as, forexample, via the Barton reduction.

In addition, the L-enantiomers corresponding to the compounds of theinvention can be prepared following the same general methods (1 or 2),beginning with the corresponding L-sugar or nucleoside L-enantiomer asthe starting material.

General Synthesis of 2′-C-Branched Nucleosides

2′-C-branched ribonucleosides of the following structures:

wherein R, R¹, R^(1′), R², R^(2′), R³, R^(3′), X, X*, and Base are allas described above, can be prepared according to Schemes 3 or 4 below.Glycosylation of the Nucleboase with an Appropriately Modified Sugar

The key starting material for this process is an appropriatelysubstituted sugar with a 2′-OH and 2′-H, with an appropriate leavinggroup (LG), such as an acyl or halogen group, for example. The sugar canbe purchased or can be prepared by any known means including standardepimerization, substitution, oxidation and/or reduction techniques. Thesubstituted sugar can then be oxidized with an appropriate oxidizingagent in a compatible solvent at a suitable temperature to yield the2′-modified sugar. Possible oxidizing agents are Jones' reagent (amixture of chromic and sulfuric acids), Collins' reagent (dipyridineCr(VI)oxide), Corey's reagent (pyridinium chlorochromate), pyridiniumdichromate, acid dichromate, potassium permanganate, MnO₂, rutheniumtetroxide, phase transfer catalysts such as chromic acid or permanganatesupported on a polymer, Cl₂-pyridine, H₂O₂-ammonium molydate, NarO₂—CAN,NaOCl in HOAc, copper chromate, copper oxide, Raney nickel, palladiumacetate, Meerwin-Pondorf-Verley reagent (aluminum t-butoxide withanother ketone) and N-bromosuccinimide.

Then coupling of an organometallic carbon nucleophile such as a Grignardreagent, an organolithium, lithium dialkylcopper or R⁶—SiMe₃ in TAF withthe ketone and an appropriate non-protic solvent at a suitabletemperature, yields the 2′-alkylated sugar. The alkylated sugaroptionally can be protected with a suitable protecting group, preferablywith an acyl or silyl group, by methods well known to those skilled inthe art, as taught by Greene et al., Protective Groups in OrganicSynthesis, John Wiley and Sons, Second Edition, 1991.

The optionally protected sugar can then be coupled to the base bymethods well known to those skilled in the art, as taught by Townsend,Chemistry of Nucleosides and Nucleotides, Plenum Press, 1994. Forexample, an acylated sugar can be coupled to a silylated base with aLewis acid, such as tin tetrachloride, titanium tetrachloride, ortrimethylsilyltriflate in an appropriate solvent at a suitabletemperature. Alternatively, a halo-sugar can e coupled to a silylatedbase in the presence of trimethylsilyltriflate.

Subsequently, the nucleoside can be deprotected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991.

In a particular embodiment, the 2′-C-branched ribonucleoside is desired,the synthesis of which is shown in Scheme 3. Alternatively, adeoxyribonucleoside is desired. To obtain these nucleosides, the formedribonucleoside can optionally be protected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991, and thenthe 2′-OH can e reduced with a suitable reducing agent. Optionally, the2′-OH can be activated to facilitate reduction, such as, for example, bythe Barton reduction.

Modification of a Pre-Formed Nucleoside

The key starting material for this process is an appropriatelysubstituted nucleoside with a 2′-OH and 2′-H. The nucleoside can bepurchased or can be prepared by any known means including standardcoupling techniques. The nucleoside optionally can be protected withsuitable protecting groups, preferably with acyl or silyl groups, bymethods well known to those skilled in the art, as described in Greeneet al., Protective Groups in Organic Synthesis, John Wiley and Sons,Second Edition, 1991.

The appropriately protected nucleoside then can be oxidized with anappropriate oxidizing agent in a compatible solvent at a suitabletemperature to yield the 2′-modified sugar. Possible oxidizing agentsinclude Jones' reagent (a mixture of chromic and sulfuric acids),Collins' reagent (dipyridine Cr(VI)oxide), Corey's reagent (pyridiniumchlorochromate), pyridinium dichromate, acid dichromate, potassiumpermanganate, MnO₂, ruthenium tetroxide, phase transfer catalysts suchas chromic acid or permanganate supported on a polymer, Cl₂-pyridine,H₂O₂-ammonium molydate, NarO₂—CAN, NaOCl in HOAc, copper chromate,copper oxide, Raney nickel, palladium acetate, Meerwin-Pondorf-Verleyreagent (aluminum t-butoxide with another ketone) andN-bromosuccinimide.

Subsequently, the nucleoside can be deprotected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991.

In a particular embodiment, a 2′-C-branched ribonucleoside is desired,the synthesis of which is shown in Scheme 4. Alternatively, thedeoxyribonucleoside may be desired. To obtain these nucleosides, theformed ribonucleoside optionally may be protected by methods well knownto those skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991, and thenthe 2′-OH can be reduced with a suitable reducing agent. Optionally, the2′-OH can be activated to facilitate reduction such as, for example, bythe Barton reduction.

In another embodiment of the invention, the L-enantiomers are desired.These L-enantiomers corresponding to the compounds of the invention maybe prepared following the same general methods given above, butbeginning with the corresponding L-sugar or nucleoside L-enantiomer asthe starting material.

C. General Synthesis of 3′-C-Branched Nucleosides

3′-C-branched ribonucleosides of the following structures:

wherein R, R¹, R^(1′), R², R^(2′), R³, R^(3′) X, X*, and Base are all asdescribed above, can be prepared according to Schemes 5 or 6 below.Glycosylation of the Nucleobase with an Appropriately Modified Sugar(Scheme 5)

The key starting material for this process is an appropriatelysubstituted sugar with a 3′-OH and a 3′-H, with an appropriate leavinggroup (LG) such as, for example, an acyl group or a halogen. The sugarcan be purchased or can be prepared by any known means includingstandard epimerization, substitution, oxidation and/or reductiontechniques. The substituted sugar then can be oxidized by an appropriateoxidizing agent in a compatible solvent at a suitable temperature toyield the 3′-modified sugar.

Possible oxidizing agents include Jones' reagent (a mixture of chromicand sulfuric acids), Collins' reagent (dipyridine Cr(VI)oxide), Corey'sreagent (pyridinium chlorochromate), pyridinium dichromate, aciddichromate, potassium permanganate, MnO₂, ruthenium tetroxide, phasetransfer catalysts such as chromic acid or permanganate supported on apolymer, Cl₂-pyridine, H₂O₂-ammonium molydate, NarO₂—CAN, NaOCl in HOAc,copper chromate, copper oxide, Raney nickel, palladium acetate,Meerwin-Pondorf-Verley reagent (aluminum t-butoxide with another ketone)and N-bromosuccinimide.

Then coupling of an organometallic carbon nucleophile such as a Grignardreagent, an organolithium, lithium dialkylcopper or R⁶—SiMe₃ in TAF withthe ketone and an appropriate non-protic solvent at a suitabletemperature, yields the 3′-C-branched sugar. The 3′-C-branched sugaroptionally can e protected with a suitable protecting group, preferablywith an acyl or silyl group, by methods well known to those skilled inthe art, as taught by Greene et al., Protective Groups in OrganicSynthesis, John Wiley and Sons, Second Edition, 1991.

The optionally protected sugar can then be coupled to the base bymethods well known to those skilled in the art, as taught in Townsend,Chemistry of Nucleosides and Nucleotides, Plenum Press, 1994. Forexample, an acylated sugar can be coupled to a silylated base with aLewis acid, such as tin tetrachloride, titanium tetrachloride, ortrimethylsilyltriflate in an appropriate solvent at a suitabletemperature. Alternatively, a halo-sugar can be coupled to a silylatedbase in the presence of trimethylsilyltriflate.

Subsequently, the nucleoside can be deprotected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991.

In a particular embodiment, the 3′-C-branched ribonucleoside is desired,the synthesis of which is shown in Scheme 5. Alternatively, adeoxyribonucleoside is desired. To obtain these nucleosides, the formedribonucleoside can optionally be protected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991, and thenthe 2′-OH can be reduced with a suitable reducing agent. Optionally, the2′-OH can be activated to facilitate reduction, such as, for example, bythe Barton reduction.

Modification of a Preformed Nucleoside.

The key starting material for this process is an appropriatelysubstituted nucleoside with a 3′-OH and 3′-H. The nucleoside can bepurchased or can be prepared by any known means including standardcoupling techniques. The nucleoside can be optionally protected withsuitable protecting groups, preferably with acyl or silyl groups, bymethods well known to those skilled in the art, as taught by Greene etal., Protective Groups in Organic Synthesis, John Wiley and Sons, SecondEdition, 1991.

The appropriately protected nucleoside can then be oxidized with theappropriate oxidizing agent in a compatible solvent at a suitabletemperature to yield the 2′-modified sugar. Possible oxidizing agentsinclude Jones' reagent (a mixture of chromic and sulfuric acids),Collins' reagent (dipyridine Cr(VI)oxide), Corey's reagent (pyridiniumchlorochromate), pyridinium dichromate, acid dichromate, potassiumpermanganate, MnO₂, ruthenium tetroxide, phase transfer catalysts suchas chromic acid or permanganate supported on a polymer, Cl₂-pyridine,H₂O₂-ammonium molydate, NarO₂—CAN, NaOCl in HOAc, copper chromate,copper oxide, Raney nickel, palladium acetate, Meerwin-Pondorf-Verleyreagent (aluminum t-butoxide with another ketone) andN-bromosuccinimide.

Subsequently, the nucleoside can be deprotected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991.

In a particular embodiment, the 3′-C-branched ribonucleoside is desired,the synthesis of which is shown in Scheme 6. Alternatively, adeoxyribonucleoside is desired. To obtain these nucleosides, the formedribonucleoside can optionally be protected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991, and thenthe 2′-OH can be reduced with a suitable reducing agent. Optionally, the2′-OH can be activated to facilitate reduction, such as, for example, bythe Barton reduction.

In another embodiment of the invention, the L-enantiomers are desired.These L-enantiomers corresponding to the compounds of the invention maybe prepared following the same general methods given above, butbeginning with the corresponding L-sugar or nucleoside L-enantiomer asthe starting material.

General Synthesis of 4′-C-Branched Nucleosides

4′-C-branched ribonucleosides of the following structures:

wherein R, R¹, R^(1′), R², R^(2′), R³, R^(3′), X, X*, and Base are allas described above, can be prepared according to the following generalmethods.Modification from the pentodialdo-furanose.

The key starting material for this process is an appropriatelysubstituted pentodialdo-furanose. The pentodialdo-furanose can bepurchased or can be prepared by any known means including standardepimerization, substitution and cyclization techniques.

In a preferred embodiment, the pentodialdo-furanose is prepared from theappropriately substituted hexose. The hexose can be purchased or can beprepared by any known means including standard epimerization (for eg.,via alkaline treatment), substitution, and coupling techniques. Thehexose can be in either the furanose form or cyclized by any means knownin the art, such as methodology taught by Townsend in Chemistry ofNucleosides and Nucleotides, Plenum Press, 1994, preferably byselectively protecting the hexose, to give the appropriate hexafuranose.

The 4′-hydroxymethylene of the hexafuranose then can be oxidized with anappropriate oxidizing agent in a compatible solvent at a suitabletemperature to yield the 4′-aldo-modified sugar. Possible oxidizingagents are Swern reagents, Jones' reagent (a mixture of chromic andsulfuric acids), Collins' reagent (dipyridine Cr(VI)oxide), Corey'sreagent (pyridinium chlorochromate), pyridinium dichromate, aciddichromate, potassium permanganate, MnO₂, ruthenium tetroxide, phasetransfer catalysts such as chromic acid or permanganate supported on apolymer, Cl₂-pyridine, H₂O₂-ammonium molybdate, NarO₂—CAN, NaOCl inHOAc, copper chromate, copper oxide, Raney nickel, palladium acetate,Meerwin-Pondorf-Verley reagent (aluminum t-butoxide with another ketone)and N-bromosuccinimide, although using H₃PO₄, DMSO and DCC in a mixtureof benzene/pyridine at room temperature is preferred.

Then the pentodialdo-furanose optionally can be protected with asuitable protecting group, preferably with an acyl or silyl group, bymethods well known to those skilled in the art, as taught by Greene etal., Protective Groups in Organic Synthesis, John Wiley and Sons, SecondEdition, 1991. In the presence of a base, such as sodium hydroxide, theprotected pentodialdo-furanose then can be coupled with a suitableelectrophilic alkyl, halogeno-alkyl (such as CF₃), alkenyl or alkynyl(i.e., allyl), to obtain the 4′-alkylated sugar. Alternatively, theprotected pentodialdo-furanose can be coupled with a correspondingcarbonyl, such as formaldehyde, in the presence of a base like sodiumhydroxide and with an appropriate polar solvent like dioxane, at asuitable temperature, and then reduced with an appropriate reducingagent to provide the 4′-alkylated sugar. In one embodiment, thereduction is carried out using PhOC(S)Cl and DMAP in acetonitrile atroom temperature, followed by reflux treatment with ACCN and TMSS intoluene.

The optionally activated sugar can be coupled to the base by methodswell known to those skilled in the art, as taught by Townsend inChemistry of Nucleosides and Nucleotides, Plenum Press, 1994. Forexample, an acylated sugar can be coupled to a silylated base with aLewis acid, such as tin tetrachloride, titanium tetrachloride, ortrimethylsilyltriflate in an appropriate solvent at room temperature.

Subsequently, the nucleoside can be deprotected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991.

In a particular embodiment, the 4′-C-branched ribonucleoside is desired.Alternatively, a deoxyribonucleoside is desired. To obtain thesenucleosides, the formed ribonucleoside can optionally be protected bymethods well known to those skilled in the art, as by Greene et al.,Protective Groups in Organic Synthesis, John Wiley and Sons, SecondEdition, 1991, and then the 2′-OH can be reduced with a suitablereducing agent. Optionally, the 2′-OH can be activated to facilitatereduction, such as, for example, by the Barton reduction.

In another embodiment of the invention, the L-enantiomers are desired.These L-enantiomers corresponding to the compounds of the invention maybe prepared following the same general methods given above, butbeginning with the corresponding L-sugar or nucleoside L-enantiomer asthe starting material.

Methods for Ribofuranosyl-2-azapurine Synthesis

Preparation of 1′-C-methyl-ribofuranosyl-2-azapurine via6-amino-9-(1-deoxy-beta-D-psicofuranosyl)purine

As an alternative method of preparation, the title compound can beprepared according to the published procedure of Farkas and Sorm (J.Farkas and F. Sorm, “Nucleic acid components and their analogues. XCIV.Synthesis of 6-amino-9-(1-deoxy-beta-D-psicofuranosyl)purine,” Collect.Czech. Chem. Commun., 1967, 32:2663-7; and J. Farkas, Collect. Czech.Chem. Commun., 1966, 31:1535 (Scheme 7).

In a similar manner, but using the appropriate sugar and 2-azapurinebase corresponding to the desired product compound, a variety of Formula(I) and/or Formula (II) compounds can be prepared.

Alternative Methods for Ribofuranosyl-Purine Analogue Synthesis

Preparation of ribofuranosyl-purine Analogues:2-aza-3,7-dideazaadenosine Derivative Compounds

Preparation of 2-aza-3,7-dideazaadenosine derivative compounds may beprepared according to the published synthesis of L. Towsend et al.,Bioorganic & Med. Chem. Letters, 1991, 1(2): 111-114, where the startingmaterial, ethyl-3-cyanopyrrole-2-carboxylate 4 was synthesized byHuisgen & Laschtuvka, according to the procedure provided in ChemischeBerichte, 1960, 93:65-81, as shown in Scheme 8:

Preparation of ribofuranosyl-purine Analogues: 2-aza-3-deazaadenosineDerivative Compounds

Preparation of 2-aza-3-deazaadenosine derivative compounds may beprepared according to the published synthesis of B. Otter et al.,J.Heterocyclic Chem., 1984, 481-89 shown in Scheme 9. The commerciallyavailable starting material used is the 4,5-dichloro-6-pyridazone 12.

An alternative preparation of 2-aza-3-deazaadenosine derivativecompounds that utilizes a chlorination step is that according to R.Panzica, J. Chem. Soc. Perkin Trans I, 1989, 1769-1774 and J.Med. Chem.,1993, 4113-4120, shown in Scheme 10:

Preparation of Purine Analogues for Nucleosides: Optionally Substituted2,8-diaza-3,7-dideazaadenine Derivative Compounds

Preparation of certain 2,8-diaza-3,7-dideazaadenosine derivativecompounds may be prepared according to the published synthesis by Oda etal. in J.Heterocyclic Chem., 1984, 21:1241-55 and Chem. Pharm. Bull.,1984, 32(11):4437-46, as shown in Scheme 11. The starting material iscommercially available 4,5-dichloro-6-pyridazone 12.

Preparation of Purine Analogues for Nucleosides:2,8-diaza-3-deazaadenosine Derivative Compounds

Preparation of certain 2,8-diaza-3-deazaadenosine derivative compoundsmay be prepared according to the published synthesis by Panzica et al.in J.Heterocyclic Chem., 1982, 285-88, J. Med. Chem., 1993, 4113-20, andBioorg. & Med. Chem. Letters., 1996, 4(10):1725-31, as provided inScheme 12. The key intermediate 27 was prepared via a 1,3-dipolarcycloaddition reaction between the 2,3,5-tri-O-benzoyl-β-D-ribofuranosylazide 26 and methyl-hydroxy-2-butylnoate 25. A ribofuranosyl azide 26synthesis was described by A. Stimac et al., Carbohydrate Res., 1992,232(2):359-65, using SnCl₄ catalyzed azidolysis of1-O-Acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose with Me₃SiN₃ in CH₂Cl₂at room temperature.

Preparation of Purine Analogues for Nucleosides: Alternative Preparationof 2,8-diaza-3-deazaadenine Derivative Compounds

2,8-diaza-3-deazaadenine derivative compounds may be prepared (seeScheme 13) according to the published synthesis by Chen et al. inJ.Heterocyclic Chem., 1982, 285-88; however, no condensation of thiscompound with ribofuranose is found.

Preparation of ribofuranosyl-2-azapurines via Use of Protective Groups

As an alternative method of preparation, the compounds of the presentinvention can also be prepared by synthetic methods well known to thoseskilled in the art of nucleoside and nucleotide chemistry, such astaught by Townsend in Chemistry of Nucleosides and Nucleotides, PlenumPress, 1994.

A representative general synthetic method is provided in Scheme 14. Thestarting material is a 3,5-is-O-protected beta-D-alkyl ribofuranoside,but it will be understood that any 2′, 3′, or 5′-position may carry aprotecting group to shield it from reacting. The 2′-C—OH then isoxidized with a suitable oxidizing agent in a compatible solvent at asuitable temperature to yield the 2′-keto-modified sugar. Possibleoxidizing agents are Swern reagents, Jones' reagent (a mixture ofchromic and sulfuric acids), Collins' reagent (dipyridine Cr(VI)oxide),Corey's reagent (pyridinium chlorochromate), pyridinium dichromate, aciddichromate, potassium permanganate, MnO₂, ruthenium tetroxide, phasetransfer catalysts such as chromic acid or permanganate supported on apolymer, Cl₂-pyridine, H₂O₂-ammonium molydate, NarO₂—CAN, NaOCl in HOAc,copper chromate, copper oxide, Raney nickel, palladium acetate,Meerwin-Pondorf-Verley reagent (aluminum t-butoxide with another ketone)and N-bromosuccinimide.

Next, addition of a Grignard reagent, such as, for example, an alkyl-,alkenyl- or alkynyl-magnesium halide like CH₃MgBr, CH₃CH₂MgBr,vinylMgBr, allylMgBr and ethynylMgBr, or an alkyl-, alkenyl- oralkynyl-lithium, such as CH₃Li, in a suitable organic solvent, such as,for example, diethyl ether or THF, across the double bond of the2′-carbonyl group provides a tertiary alcohol at this position. Theaddition of a hydrogen halide in a suitable solvent, such as, forexample, Hr in HOAc, in the subsequent step provides a leaving group(LG) such as, for example, a chloro, bromo or iodo, at the C-1 anomericcarbon of the sugar ring that later generates a nucleosidic linkage.Other suitable LGs include C-1 sulfonates such as, for example,methanesulfonate, trifluoromethanesulfonate and/or p-toluenesulfonate.

The introduction in the next step of a metal salt (Li, Na or K) of anappropriately substituted 2-azapurine in a suitable organic solvent suchas, for example, THF, acetonitrile of DMF, results in the formation ofthe desired nucleosidic linkage and addition of the desired 2-azapurinebase. This displacement reaction may be catalyzed by a phase transfercatalyst like TDA-1 or triethylbenzylammonium chloride. The introductionof a “Z” substituent on any of base formulae (i)-(vi) optionally may beperformed subsequent to the initial addition of protecting groups. Forexample, the introduction of an amino group for “Z” is accomplished bythe addition of an appropriate amine in an appropriate solvent to the2′-C-halo intermediate just prior to the last step of removal of theprotecting groups. Appropriate amines include alcoholic or liquidammonia to generate a primary amine (—NH₂), an alkylamine to generate asecondary amine (—NHR), or a dialkylamine to generate a tertiary amine(—NRR′).

Finally, the nucleoside can be deprotected by methods well known tothose skilled in the art, as by Greene et al., Protective Groups inOrganic Synthesis, John Wiley and Sons, Second Edition, 1991. It is tobe noted that this reaction scheme can be used for joining any of thepurine nucleoside analogue bases provided for in Schemes 8-13 with aribofuranosyl moiety.

The present invention is described by way of illustration in thefollowing examples. It will be understood by one of ordinary skill inthe art that these examples are in no way limiting and that variationsof detail can be made without departing from the spirit and scope of thepresent invention.

EXAMPLES

The test compounds were dissolved in DMSO at an initial concentration of200 μM and then were serially diluted in culture medium.

Unless otherwise stated, bay hamster kidney (HK-21) (ATCC CCL-10) andbos Taurus (T) (ATCC CRL 1390) cells were grown at 37° C. in ahumidified CO₂ (5%) atmosphere. HK-21 cells were passaged in Eagle MEMadditioned of 2 mM L-glutamine, 10% fetal ovine serum (FS, Gibco) andEarle's SS adjusted to contain 1.5 g/L sodium bicarbonate and 0.1 mMnon-essential amino acids. T cells were passaged in Dulbecco's modifiedEagle's medium with 4 mM L-glutamine and 10% horse serum (HS, Gibco),adjusted to contain 1.5 g/L sodium bicarbonate, 4.5 g/L glucose and 1.0mM sodium pyruvate. The vaccine strain 17D (YFV-17D) (Stamaril®, PasteurMerieux) and Bovine Viral Diarrhea virus (BVDV) (ATCC VR-534) were usedto infect HK and T cells, respectively, in 75 cm² bottles. After a 3 dayincubation period at 37° C., extensive cytopathic effect was observed.Cultures were freeze-thawed three times, cell debris were removed bycentrifugation and the supernatant was aliquoted and stored at −70° C.YFV-17D and VDV were titrated in HK-21 and T cells, respectively, thatwere grown to confluency in 24-well plates.

The following examples are derived by selection of an appropriate,optionally substituted sugar or cyclopentane ring coupled with anoptionally substituted 2-azapurine base, and prepared according to thefollowing synthetic schemes:

-   -   Example 1: Synthesis of optionally substituted        1′-C-branched-ribofuranosyl, -sulfonyl, -thiophenyl or        cyclopentanyl-2-azapurines;    -   Example 2: Synthesis of optionally substituted        2′-C-branched-ribofuranosyl, -sulfonyl, -thiophenyl or        cyclopentanyl-2-azapurines;    -   Example 3: Synthesis of optionally substituted        3′-C-branched-ribofuranosyl, -sulfonyl, -thiophenyl or        cyclopentanyl-2-azapurines;    -   Example 4: Synthesis of optionally substituted        4′-C-branched-ribofuranosyl, -sulfonyl, -thiophenyl or        cyclopentanyl-2-azapurines;    -   Examples 5-13: Synthesis of specific compounds of the present        invention; and    -   Examples 14-18: Biologic test results of representative examples        of compounds of the present invention.

Example 1 1′-C-Branched ribofuranosyl, -sulfonyl orcyclopentanyl-2-azapurine, Optionally Substituted

The title compound is prepared according to Schemes 1, 2, or 7. In asimilar manner but using the appropriate sugar or cyclopentane ring andoptionally substituted 2-azapurine base, the following nucleosides ofFormulae (I) or (II) may be prepared:

wherein: base may be any of the Formulae (A)-(G) as described hereinwhere R in each instance may exist in mono-, di- or triphosphate form.

Alternatively, the Dimroth rearrangement may be used for making2-azapurines from the corresponding purine base. In this reaction, anN-alkylated or N-arylated imino heterocycle undergoes rearrangement toits corresponding alkylamino or arylamino heterocycle.

Example 1a 1′-C-hydroxymethyl-2-azaadenosine

Step 1: 2-azaadenine, NaH, ACN, rt, 24 hrs; Step 2: MeONa/MeOH

The starting material 2-azaadenine may be prepared starting frommalonitrile by the synthesis taught by D. W. Wooley, Journal ofBiological Chemistry, (1951), 189:401.

Example 2 2′-C-Branched ribofuranosyl, -sulfonyl orcyclopentanyl-2-azapurine, Optionally Substituted

The title compound is prepared according to Schemes 3, 4, or throughprotection of appropriately selected substituent groups in Schemes 7 or8. In a similar manner but using the appropriate sugar or cyclopentanering and optionally substituted 2-azapurine base, the followingnucleosides of Formulae (I) or (II) may be prepared:

wherein: base may be any of the Formulae (A)-(G) as described hereinwhere R in each instance may exist in mono-, di- or triphosphate form.

Alternatively, the Dimroth rearrangement may be used for making2-azapurines from the corresponding purine base. In this reaction, anN-alkylated or N-arylated imino heterocycle undergoes rearrangement toits corresponding alkylamino or arylamino heterocycle.

Example 2a 2′-C-methyl-2-azaadenosine

(Synthesis according to the procedure of J. A. Montgomery, Nucleic AcidChemistry, 1978, Part II, 681-685 starting with 2′-C-methyladenosine.)

Step 1: H₂O₂, AcOH, 80%; Step 2: BnBr, DMAc, Step 3: NaOH, H₂O, EtOH,30%, Step 4: NH₃/MeOH, 80° C., 2 days, 60%; Step 5: H₂/Pd/C, 3 atm,MeOH, 30% Step 6: .NaNO₂, AcOH, H₂O, 50%.

4-Amino-7-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]-ν-triazine(2′-C-methyl-2-azaadenosine): ¹H NMR (DMSO-d₆) δ 8.82 (s, 1H, H8), 7.97(br, 2H, NH₂), 6.12 (s, 1H, H1′), 5.22-5.51 (m, 3H, 3OH), 3.70-4.17 (m,4H, H3′, H4′, 2H5′), 0.80 (s, 3H, CH₃).

¹³C NMR (DMSO-d₆) δ 153, 146, 142, 116, 92, 83, 79, 72, 60, 20. m/z(FAB>0) 565 (2M+H)⁺, 283 (M+H)⁺, (FAB<0) 563 (2M−H)^(−.)

Alternatively, 2-azaadenosine shown as the final product in Example 2.a.may be prepared starting with adenosine, according to the procedure ofJ. A. Montgomery, Nucleic Acid Chemistry, 1978, Part II, 681-685starting with 2′-C-methyladenosine, or via 2-azainosine in a syntheticprocedure taught by R. P. Panzica, Journal of Heterocyclic Chemistry,1972, 9:623-628 starting with AICA riboside.

Example 2b 2′-C-methyl-pyrrolo-4-amino-1,2,3-triazine

Step 1: NCS, DMF; Step 2: mcPBA, AcOH; Step 3: a) BnBr, DMAc; b) NaOH,H₂O, EtOH, Step 4: NH₃/MeOH, 80° C., Step 5: H₂/Pd/C, MeOH; Step 6:NaNO₂, AcOH, H₂O.

Example 3 3′-C-Branched ribofuranosyl, -sulfonyl orcyclopentanyl-2-azapurine, Optionally Substituted

The title compound is prepared according to Schemes 5, 6, or throughprotection of appropriately selected sustituent groups in Scheme 8. In asimilar manner ut using the appropriate sugar or cyclopentane ring andoptionally substituted 2-azapurine base, the following nucleosides ofFormulae (I) and (II)may be prepared:

wherein: base may be any of the Formulae (A)-(G) as described hereinwhere R in each instance may exist in mono-, di- or triphosphate form.

Alternatively, the Dimroth rearrangement may be used for making2-azapurines from the corresponding purine base. In this reaction, anN-alkylated or N-arylated imino heterocycle undergoes rearrangement toits corresponding alkylamino or arylamino heterocycle.

Example 4 4′-C-Branched ribofuranosyl, -sulfonyl orcyclopentanyl-2-azapurine, Optionally Substituted

The title compound is prepared according to modification from thecorresponding pentodialdo-furanose. In a similar manner but using theappropriate sugar or cyclopentane ring and optionally substituted2-azapurine base, the following nucleosides of Formulae (I) or (II) maybe prepared:

wherein: base may be any of the Formulae (A)-(G) as described hereinwhere R in each instance may exist in mono-, di- or triphosphate form.

Alternatively, the Dimroth rearrangement may be used for making2-azapurines from the corresponding purine base. In this reaction, anN-alkylated or N-arylated imino heterocycle undergoes rearrangement toits corresponding alkylamino or arylamino heterocycle.

Example 5 Synthesis of4-amino-1-(□-D-ribofuranosyl)imdazo[4,5-d]pyridazine

Step A:1-(2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)-5-benzyloxymethylimidazo[4,5-d]pyridazin-4-one

The 5-benzyloxymethylimidazo[4,5-d]pyridazine (500 mg, 1.95 mmol) [forpreparation see Journal of Heterocyclic Chemistry, 1984, Vol 21, 481]was heated at reflux in hexamethyldisilazane (6 mL) for 1 hour. Themixture was evaporated to dryness to give a slight yellow syrup whichwas dissolved in dry 1,2-dichloroethane (20 mL). The 1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose (1.04 g, 2.06 mmol) and stannicchloride (0.4 mL, 3.44 mmol) were added at 20° C. and the mixture wasstirred for 3 hours. The reaction mixture was poured into an aqueoussolution of sodium hydrogenocarbonate, filtrated through a pad of celiteand washed by dichloromethane. The organic layer was evaporated todryness to give a yellow foam. The crude product was purified on silicagel using n-hexane/ethyl acetate (3/2) as eluant to give the titlecompound (703 mg) as a white powder.

¹H NMR (DMSO-d₆) δ ppm: 4.39 (s, 2H, CH₂), 4.60 (m, 2H), 4.73 (m, 1H),5.34 (dd, 2H, CH₂), 5.77-5.88 (m, 2H, H2′ and H3′), 6.56 (m, 1H, H1′),6.98-7.10 (m, 5H), 7.23-7.32 (m, 6H), 7.41-7.51 (m, 3H), 7.68-7.73 (m,2H), 7.74-7.8 (m, 4H), 8.51 (s, 1H), 8.52 (s, 1H).

Step B:1-(2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazin-4-one

To a solution containing the compound from Step B (500 mg, 0.7 mmol), indry dichloromethane (25 mL) was added a pre-cooled (−78° C.) solution ofboron trichloride 1M (5 mL) at −78° C. and stirred for 2 hours at −78°C. A mixture of methanol/dichloromethane (1/1) was added to the mixtureat −78° C. and then at 20° C. The reaction mixture was evaporated todryness to give a yellow powder. The crude product was purified onsilica gel using n-hexane/ethyl acetate (3/2) as eluant to give thetitle compound (400 mg) as a yellow powder.

¹H NMR (DMSO-d₆) δ ppm: 4.77-4.98 (m, 3H, H4′, 2H5′), 5.95-6.12 (m, 2H,H2′ and H3′), 6.65 (m, 1H, H1′), 7.39-7.76 (m, 9H), 7.84-8.06 (m, 6H),8.64-5.79 (m, 2H, H3 and H8), 12.84 (br, 1H, NH).

Mass spectrum: m/z (FAB>0) 581 (M+H)⁺, (FAB<0) 579 (M−H)⁻

Step C:4-chloro-1-(2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

A solution containing the compound from Step B (1.32 g, 2.27 mmol), theN,N-diethylaniline (365 μL), tetrabutylammonium chloride (1.2 g),freshly distilled phosphorus chloride (1.3 μL) and anhydrousacetonitrile (17 mL) was stirred at 90° C. for 1 hour. The reactionmixture was poured over cracked ice/water. The aqueous layer wasextracted with dichloromethane (3×60 mL). The organic layer was washedwith sodium hydrogenocarbonate 5%, water and was evaporated to dryness.The crude product was purified on silica gel using n-hexane/ethylacetate (3/1) as eluant to give the title compound (404 mg) as a yellowpowder.

¹H NMR (DMSO-d₆) δ ppm: 4.82-6.87 (m, 2H), 4.9-6.95 (m, 1H), 6.0-6.08(m, 1H), 6.12-6.19 (m, 1H), 6.90 (d, 1H, J=5.2 Hz, H1′), 7.47-7.73 (m,9H), 7.88-8.12 (m, 6H), 9.10 (s, 1H, H8), 9.90 (s, 1H, H3)

Step D: 4-amino-1-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

The compound from Step C (420 mg, 0.7 mmol) was added to a solution ofammonia in methanol and stirred in a steel bomb at 150° C. for 6 hours.The reaction mixture was evaporated to dryness to afford a brown oilwhich was purified on silica gel reverse-phase (C18) using water aseluant to give the title compound (50 mg) as a yellow powder.

¹H NMR (DMSO-d₆) δ ppm: 3.58-4.48 (m, 5H, H2′, H3′, H4′, 2H5′),5.14-5.68 (m, 3H, 3×OH), 5.90 (s, 1H, H1′), 6.61 (br, 2H, NH₂), 8.59 (s,1H, H8), 9.12 (s, 1H, 3H)

Example 6 Synthesis of1-(β-D-ribofuranosyl)imidazo[4,5-d]pyridazin-4-one

1-(2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazin-4-one(555 mg, 0.9 mmol) was added to a solution of sodium methylate (205 mg)in methanol (25 mL) and stirred at 20° C. for 2 hours. The reactionmixture was evaporated to dryness. The residue was dissolved in waterand washed with ethyl acetate. The aqueous layer was concentrated underpressure. The crude product was purified on silica gel reverse-phase(C18) using water as eluant to give the title compound (220 mg) as awhite powder.

¹H NMR (DMSO-d₆) δ ppm: 3.59-3.62 (m, 2H), 4.02 (m, 1H), 4.11 (m, 1H),4.22 (m, 1H), 5.16-5.72 (m, 3H, 3×OH), 5.91 (s, 1H, H1′), 8.52 (s, 1H,H8), 8.68 (s, 1H, H3), 12.75 (br, 1H, NH).

Mass spectrum: m/z (FAB>0) 537 (2M+H)⁺, 269 (M+H)⁺, (FAB<0) 535 (2M+H)⁺,267 (M−H)⁻

Example 7 Synthesis of4-amino-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

Step A:1-(2-C-methyl-2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazin-4-one

To a suspension of imidazo[4,5-d]pyridazine (3.48 g, 25.5 mmol) [forpreparation see Journal of Heterocyclic Chemistry, 1969, Vol 6, 93] indry acetonitrile (35 mL) was added1,2,3,5-tetra-O-benzoyl-2-C-methyl-β-D-ribofuranose (14.48 g, 25.0 mmol)at 20° C. and stirred for 15 mn. DBU (11.5 mL, 76.3 mmol) was added at0° C. and the solution was stirred for 15 mn at 0° C. TMSOTf (24.7 mL,127.8 mmol) was added at 0° C. and the mixture was heated at 80° C. for20 hours. The reaction mixture was poured into an aqueous solution ofsodium hydrogenocarbonate and extracted by ethyl acetate. The organiclayer was evaporated to dryness to give a yellow powder. The crudeproduct was purified on silica gel using dichloromethane/methanol(99.3/0.7) as eluant to give a slight yellow powder which wascrystallized from isopropanol to give the title compound (2.45 g) as awhite powder.

¹H NMR (DMSO-d₆) δ ppm: 1.48 (s, 3H, CH₃), 4.75-4.96 (m, 3H, H4′, 2H5′),5.81 (d, 1H, J=5.5 Hz, H3′), 6.99 (s, 1H, H1′), 7.39-7.72 (m, 9H),7.92-8.08 (m, 6H), 8.64 (s, 1H, H8), 8.71 (s, 1H, H3), 12.89 (br, 1H,NH).

Mass spectrum: m/z (FAB>0) 1189 (2M+H)⁺, 585 (M+H)⁺, (FAB<0) 593 (M−H)⁻

Step B:4-chloro-1-(2-C-methyl-2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

A solution containing the compound from Step A (300 mg, 0.50 mmol), theN,N-diethylaniline (1.2 mL) and freshly distilled phosphorus chloride(24 mL) was stirred at reflux for 1 hour. The reaction mixture wasevaporated to dryness. Dichloromethane was added to the residue and theorganic layer poured over cracked ice/water. The aqueous layer wasextracted with dichloromethane. The organic layer was washed with sodiumhydrogenocarbonate 5%, water and was evaporated to dryness. The crudeproduct was purified on silica gel using diethyl ether/petrol ether(1/1) as eluant to give the title compound (295 mg) as a white powder.

¹H NMR (DMSO-d₆) δ ppm: 1.5 (s, 3H, CH₃), 4.8-5.0 (m, 3H, H4′, 2H5′),5.85 (d, 1H, J=5.5 Hz, H3′), 7.15 (s, 1H, H1′), 7.38-8.08 (m, 15H), 9.15(s, 1H, H8), 9.90 (s, 1H, H3)

Step C: 4-amino-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

The compound from Step B (590 mg, 0.96 mmol) was added to a solution ofammonia in methanol and stirred in a steel bomb at 150° C. for 6 hours.The reaction mixture was evaporated to dryness to remove methanol. Thecrude product was purified on silica gel reverse-phase (C18) using wateras eluant to give the title compound (35 mg) as a white powder.

¹H NMR (DMSO-d₆) δ ppm: 0.70 (s, 3H, CH₃), 3.64-3.98 (m, 4H, H3′, H4′,2H5′), 5.23-5.44 (m, 3H, 3OH), 5.98 (s, 1H, H1′), 6.63 (br, 2H, NH₂),8.68 (s, 1H, H8), 9.05 (s, 1H, H3)

¹³C NMR (DMSO-d₆) δ ppm: 155, 143, 132, 131, 129, 93, 83, 79, 72, 20.

Mass spectrum: m/z (FAB>0) 282 (M+H)⁺, (FAB<0) 280 (M−H)⁻

Example 8 Synthesis of the4-substituted-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5d]pyridazine

W Step A products OH NaOMe, MeOH 100° C., 24 h

Cl NH₃/MeOH 20° C., 48 h

1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazin-4-one

¹H NMR (DMSO-d₆) δ ppm: 1.17 (s, 3H, CH₃), 3.44-3.59 (m, 1H), 3.68-3.78(m, 1H), 3.86-3.94 (m, 1H), 4.11-4.21 (m, 1H), 4.8-5.4 (m, 3H, 3OH),6.05 (s, 1H, H1′), 8.35 (s, 1H, H8), 8.37 (s, 1H, H3), 12.67 (br, 1H,NH).

Mass spectrum: m/z (FAB>0) 283 (2M+H)⁺, 281 (M+H)⁺

4-chloro-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 0.72 (s, 3H, CH₃), 3.69-4.06 (m, 4H, H3′, H4′,2H5′), 5.34-5.51 (m, 3H, 3OH), 6.19 (s, 1H, H1′), 9.18 (s, 1H, H8), 9.87(s, 1H, H3)

Mass spectrum: m/z (FAB>0) 301 (M+H)⁺, (FAB<0) 299 (M−H)⁻

Example 9 Synthesis of the 4,7-diamino-imidazo[4,5-d]pyridazinenucleosides Derivatives

Step A:

To a suspension of 4,5-dicyanoimidazole (1 eq.) [for preparation seeJournal of Organic Chemistry, 1976, Vol 41, 713] in dry DMF (0.2 M) wasadded the protected β-D-ribofuranose derivatives (1 eq.) at 20° C. DBU(3 eq.) was added at 0 C and the solution was stirred for 20 mn at 0° C.TMSOTf (4 eq.) was added at 0° C. and the mixture was heated at 60° C.for 1 hour. The reaction mixture was poured into an aqueous solution ofsodium hydrogenocarbonate and extracted by dichloromethane. The organiclayer was evaporated to dryness to give a yellow powder. The crudeproduct was purified on silica gel using diethyl ether/petrol ether aseluant to give the title compound (see the following table 1).

Step B:

The compound from Step A (1 eq.) was stirred with hydrazine monohydrate(20 eq.) and acetic acid (1.4 eq.) at 75° C. for several hours (see thefollowing table 1). The reaction mixture was poured into water. Theaqueous layer was washed by dichloromethane and evaporated underpressure. The residue was purified on reverse-phase column to give thetitle compound (see the following table 1). TABLE 1 R products from StepA yield (Step A) experiments (Step B) products from Step B yield (StepA) H

63% 75° C. for 1.5 h

58% (white powder) CH₃

62% 75° C. for 20 h.

15% (white powder)

4,7-diamino-1-β-D-ribofuranosylimidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 3.58-4.32 (m, 5H, H2′, H3′, H4′, 2H5′),5.10-5.90 (br, 7H, 2NH₂, 3OH), 6.11 (s, 1H, H1′), 8.50 (s, 1H, H8)

¹³C NMR (DMSO-d₆) δ ppm: 151, 144, 142, 132, 122, 89, 86, 75, 70, 61.

Mass spectrum: m/z (FAB>0) 283 (M+H)⁺, (FAB<0) 281 (M−H)⁻

4,7-diamino-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 0.75 (s, 3H, CH₃), 3.67-3.76 (m, 1H), 3.84-3.94(m, 3H), 5.32 (m, 3H, 3OH), 5.43 (br, 1H, NH₂), 5.71 (br, 1H, NH₂), 6.21(s,1H, H1′), 8.78 (s, 1H, H8)

¹³C NMR (DMSO-d₆) δ ppm: 151, 144, 142, 132, 123, 92, 83, 78, 71, 59,20.

Mass spectrum: m/z (FAB>0) 593 (2M+H)⁺, 297 (M+H)⁺, (FAB<0) 295 (M−H)⁻

Example 10 Synthesis of 4,7-disubstituted-imidazo[4,5-d]pyridazinenucleosides

Step A: Typical Procedure for the Preparation of the protected4,7-dichloroimidazo[4,5-d]pyridazine nucleosides

The 4,7-dichloroimidazo[4,5-d]pyridazine [for preparation see Journal ofHeterocyclic Chemistry, 1968, Vol 5, 13] (1 eq.) was heated at reflux inhexamethyldisilazane for 12 hours. The mixture was evaporated to drynessto give a solid which was dissolved in 1,2-dichloroethane. The protectedβ-D-ribofuranose derivatives (1.1 eq.) and stannic chloride (1.4 eq.)were added at 20° C. and the solution was stirred for 3 hours. Thereaction mixture was poured into an aqueous solution of sodiumhydrogenocarbonate, filtrated through a pad of celite and washed bydichloromethane. The organic layer was evaporated to dryness. The crudeproduct was purified on silica gel using dichloromethane/acetone (40/1)as eluant to give the title compound (see the following table 2).

Step B: Typical Procedure for the Preparation of the4,7-dichloroimidazo[4,5-d]pyridazine nucleosides

The compound from Step A (1 eq.) was stirred with sodium methoxide (0.1eq.) in methanol for several hours. The reaction mixture was evaporatedunder pressure. Water was added to the residue. The aqueous layer waswashed by ethyl acetate and was evaporated under pressure. The residuewas purified on reverse-phase column to give the title compound (see thefollowing table 2).

Step C: Typical Procedure for the Preparation of theimidazo[4,5-d]pyridazine nucleosides

A mixture of the compound from Step A (1 eq.), palladium on charcoal(10%), sodium acetate (4.2 eq.) in acetyl acetate was stirred underhydrogen until the compound from Step A was consumed. The reactionmixture was evaporated under pressure and was purified on silica gel togive the title protected compound which was stirred with sodiummethoxide (3.3 eq.) in methanol. The reaction mixture was evaporatedunder pressure. Water was added to the residue. The aqueous layer waswashed by ethyl acetate and was evaporated under pressure. The residuewas purified on reverse-phase column to give the title compound (see thefollowing table 2).

Step D: Typical Procedure for the Preparation of thechloro-methoxy-imidazo[4,5-d]pyridazine nucleosides

The compound from Step A (1 eq.) was stirred with sodium methoxide (3.3eq.) in methanol 0.3M at 20° C. for several hours. The reaction mixturewas evaporated under pressure. Water was added to the residue. Theaqueous layer was washed by ethyl acetate and was evaporated underpressure. The residue was purified on reverse-phase column to give acompound whose regioselectivity was not given (see the following table2).

Step E: Typical Procedure for the Preparation of themethoxy-imidazo[4,5-d]pyridazine nucleosides

A mixture of the compound from Step D (1 eq.), palladium on charcoal(10%), sodium acetate (4.2 eq.) in water/ethanol (1/1) was stirred underhydrogen until the compound from Step A was consumed. The reactionmixture was evaporated under pressure and was purified on reverse-phasecolumn to give the title compound whose regioselectivity was not given(see the following table 2).

Step F: Typical Procedure for the Preparation of the Protected4,7-diazidoimidazo[4,5-d]pyridazine nucleosides

The compound from Step A (1 eq.) was treated at 50° C. with sodium azide(1.5 eq.) in DMF. Water was added to the mixture. The aqueous layer wasextracted by ethyl acetate. The organic layer was evaporated underpressure. The crude product was purified on silica gel using diethylether/petrol ether (7/3) as eluant to give the title compound (see thefollowing table 2).

Step G: Typical Procedure for the Preparation of theazido-methoxy-imidazo[4,5-d]pyridazine nucleosides

The compound from Step F (1 eq.) was stirred at 50° C. with sodiummethoxide (1 eq.) in methanol. The reaction mixture was evaporated underpressure. Water was added to the residue. The aqueous layer was washedby ethyl acetate and was evaporated under pressure. The residue waspurified on reverse-phase column using water/acetonitrile as eluant togive the title compound whose regioselectivity was not given (see thefollowing table 2).

Step H: Typical Procedure for the Preparation of theamino-azido-imidazo[4,5-d]pyridazine nucleosides

A mixture of the compound from Step F (1 eq.), palladium on charcoal(10%), sodium acetate (4.2 eq.) in ethyl acetate was stirred underhydrogen until the compound from Step F was consumed. The reactionmixture was filtrated over celite and was evaporated under pressure Thecrude product was purified on silica gel to give the title protectedcompound whose regioselectivity was not given (see the following table1). This compound was stirred with sodium methoxide (3 eq.) in methanol.The reaction mixture was evaporated under pressure. Water was added tothe residue. The aqueous layer was washed by ethyl acetate and wasevaporated under pressure. The residue was purified on reverse-phasecolumn to give the title compound whose regioselectivity was not given(see the following table 2). TABLE 2 experi- ments products yield Step A

60% (white powder) Step A

34% (white powder) Step B

     (white powder) Step C

71% (white powder) Step D

55% (white powder) Step E

55% (white powder) Step F

50% (yellow powder) Step F

50% (yellow powder) Step G

36% (beige powder) Step H

98% (white powder) Step H

49% (white powder)

4,7-dichloro-1-(2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 4.8-5.0 (m, 3H, H4′, 2H5′), 6.05 (s, 1H, H3′),6.25 (s, 1H, H2′), 7.1 (d, 1H, J=4 Hz, H1′), 7.4-8.0 (m, 15H), 9.25 (s,1H, H8).

Mass spectrum: m/z (FAB>0) 633 (M+H)⁺

4,7-dichloro-1-(2-C-methyl-2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 1.65 (s, 3H, CH₃), 4.9-5.0 (m, 3H, H4′, 2H5′),5.8 (s, 1H, H3′), 7.35-8.05 (m, 16H including H1′), 9.3 (s, 1H, H8).

4,7-dichloro-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 0.84 (s, 3H, CH₃), 3.77 (m, 1H), 3.88-4.04 (m,3H), 5.30-5.60 (m, 3H, OH), 6.5 (s, 1H, H1′), 9.44 (s, 1H, H8).

1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 0.80 (s, 3H, CH₃), 3.75 (m, 1H), 3.80-4.00 (m,3H), 5.40 (br, 3H, OH), 6.2 (s, 1H, H1′), 9.0 (s, 1H), 9.65 (s, 1H),9.85 (s, 1H).

7-chloro-4-methoxy-1-β-D-ribofuranosylimidazo[4,5-d]pyridazine or4-chloro-7-methoxy-1-β-D-ribofuranosylimidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 3.6-4.5 (m, 8H, 2H5′, H4′, H3′, H2′, OCH₃), 5.40(m, 3H, OH), 6.2 (s, 1H, H1′), 9.0 (s, 1H, H8).

675 (2M+H)⁺, Mass spectrum: m/z (FAB>0) 317 (M+H)⁺, (FAB<0) 315 (M−H)⁻

4-methoxy-1-β-D-ribofuranosylimidazo[4,5-d]pyridazine or7-methoxy-1-β-D-ribofuranosylimidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 3.54-3.79 (m, 2H), 3.95 (m, 1H), 4.15 (m, 1H,H3′), 4.2 (s, 3H, OCH₃), 4.4 (m, 1H, H2′), 5.05-5.70 (m, 3H, OH), 6.2(d, 1H, J=4.8 Hz, H1′), 8.95 (s, 1H), 9.3 (s, 1H).

¹³C NMR (DMSO-d₆) δ ppm: 154, 145, 143, 142, 121, 90, 85, 75, 70, 61,55.

Mass spectrum: m/z (FAB>0) 283 (M+H)⁺, (FAB<0) 281 (M−H)⁻

4,7-diazido-1-(2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 4.81-5.1 (m, 3H, H4′, 2H5′), 6.24-6.49 (m, 2H,H2′, H3′), 7.2 (d, 1H, J=5 Hz, H1′), 7.4-8.0 (m, 15H), 9.12 (s, 1H, H8).

Mass spectrum: m/z (FAB>0) 647 (M+H)⁺

4,7-diazido-1-(2-C-methyl-2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 1.6 (s, 3H, CH₃), 4.96 (m, 3H, H4′, 2H5′), 6.02(m, 1H, H3′), 7.24 (s, 1H, H1′), 7.40-7.52 (m, 6H), 7.60-7.71 (m, 3H),7.93-8.1 (m, 6H), 9/10 (s, 1H, H8).

Mass spectrum: m/z (FAB>0) 661 (M+H)⁺

4-azido-7-methoxy-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazineor7-azido-4-methoxy-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 0.75 (s, 3H, CH₃), 3.70-3.98 (m, 2H), 4.05 (m,2H), 4.2 (s, 3H, OCH₃), 5.32-5.61 (br, 3H, OH), 6.36 (d, 1H, J=5.7 Hz,H1′), 9.18 (s, 1H, H8),

¹³C NMR (DMSO-d₆) 8 ppm: 156, 143, 136, 129, 123, 94, 83, 79, 71, 59,57, 20.

Mass spectrum: m/z (FAB>0) 675 (2M+H)⁺, 338 (M+H)⁺, (FAB<0) 673 (2M−H)⁻,336 (M−H)

4-amino-7-azido-1-(2-C-methyl-2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazineor7-amino-4-azido-1-(2-C-methyl-2,3,5-tri-O-Benzoyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 01.64 (s, 3H, CH₃), 4.95 (m, 3H), 6.06 (m, 1H,H3′), 7.18 (s, 1H, H1′), 7.40-7.52 (m, 6H), 7.63-7.74 (m, 5H, includingNH₂), 7.91-8.04 (m, 6H), 8.96 (s, 1H, H8),

Mass spectrum: m/z (FAB>0) 1269 (2M+H)⁺, 635 (M+H)⁺, (FAB<0) 633 (M−H)⁻

4-amino-7-azido-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazineor7-amino-4-azido-1-(2-C-methyl-β-D-ribofuranosyl)imidazo[4,5-d]pyridazine

¹H NMR (DMSO-d₆) δ ppm: 0.75 (s, 3H, CH₃), 3.65-4.15 (m, 4H), 5.30-5.55(br, 3H, 3×OH), 6.27 (s, 1H, H1′), 7.63 (br, 2H, NH₂), 9.03 (s, 1H, H8),

Mass spectrum: m/z (FAB>0) 323 (M+H)⁺, (FAB<0) 321 (M−H)

Example 11 Synthesis of 4-amino-6-substituted-imidazo[4,5-d]-ν-triazinenucleosides

Step A: 4-amino-6-bromo-7-(β-D-ribofuranosyl)imidazo[4,5-d]-ν-triazine

The 2-azaadenosine [for preparation see Patent WO 01/16149. 2001] (70mg, 0.26 mmol) was added to a solution of sodium acetate 0.5M (1.4 mL).The solution was heated until the 2-azaadenosine was solubilized. Asolution of bromine (100 μL of Br₂ in 10 mL of water) (6.3 mL, 1.22mmol) was added and the mixture was stirred at 20° C. for 3 days. Asecond portion of the bromine's solution (6.3 mL, 1.22 mmol) was addedand the mixture was stirred at 20° C. for 3 hours. The reaction mixturewas evaporated to dryness. The crude product was purified on silica gelreverse-phase (C18) using water/acetonitrile (9/1) as eluant to give thetitle compound as a yellow powder.

¹H NMR (DMSO-d₆) δ ppm: 3.55 (m, 1H, H₅′), 3.71 (m, 1H, H₅′), 4.01 (m,1H, H₄′), 4.31 (m, 1H, H₃′), 5.17 (m, 1H, H₂′), 5.19 (m, 1H, OH), 5.36(m, 1H, OH), 5.58 (m, 1H, OH), 5.93 (d, 1H, J=6.47 Hz, H1′), 8.08 (br,2H, NH₂).

Mass spectrum: m/z (FAB>0) 349 (M+2H)⁺, m/z (FAB<0) 345 (M−2H)⁻.

Step B: 4-amino-6-methyl-7-(β-D-ribofuranosyl)imidazo[4,5-d]-ν-triazine

The compound from Step A (112 mg, 0.3 mmol) was heated at reflux inhexamethyldisilazane (15 mL) for 16 hours. The mixture was evaporated todryness to give a syrup which was dissolved in dry THF (12 mL). PPh₃ (10mg; 0.04 mmol), PdCl₂ (3.5 mg; 0.02 mmol) and AlMe₃ (100 μl; 0.94 mmol)were added. The mixture was reflux for 5 hours. The mixture wasevaporated to dryness. The crude product was dissolved in methanol (30mL) in the presence of ammonium chloride. The mixture was evaporated todryness and the residue was purified on silica gel reverse-phase (C18)using water/acetonitrile (from 9/1 to 6/4) as eluant to give the titlecompound (35 mg) as a yellow powder.

¹H NMR (DMSO-d₆) δ ppm: 2.67 (s, 3H, CH₃), 3.60 (m, 1H, H₅′), 3.72 (m,1H, H₅′), 4.03 (m, 1H, H₄′), 4.24 (m, 1H, H₃′), 4.93 (m, 1H, H₂′), 5.48(m, 3H, OH), 5.92 (d, 1H, J=6.82 Hz, H₁′), 7.78 (br, 2H, NH₂).

Mass spectrum: m/z (FAB>0) (FAB>0) 283 (M+H)⁺, m/z (FAB<0) 281 (M−H)⁻.

Example 12 Synthesis of imidazo[4,5d]-triazin-4-one nucleosides

Step A: 7-(β-D-Ribofuranosyl)imidazo[4,5-d]-ν-triazin-4-one

The AICAR [for preparation see Synthesis, 2003, No 17, 2639] (1 g, 3.87mmol) was added to a solution of chlorhydrique acid 6N (25 mL) at −30°C. A solution of sodium nitrite 3M (4 ml, 11.62 mmol) was added and themixture was stirred at −30° for 2 hours. A pre-cooled (−30° C.) solutionof ethanol (25 mL) was added. A solution of ammonia (28%) was added at−20° C. to pH=7. The reaction mixture was evaporated to dryness. Thecrude product was purified on silica gel reverse-phase (C18) using wateras eluant to give the title compound (0.81 gr) as a white powder.

¹H NMR (DMSO-d₆) δ ppm: 3.58 (d, 1H, J=11.85 Hz, H₅′), 3.70 (d, 1H,J=11.85 Hz, H₅′), 4.00 (dd, 1H, J=3.92 Hz, 4.02 Hz, H4′), 4.18 (dd, 1H,J=4.27 Hz, 4.78 Hz, H₃′), 4.54 (dd, 1H, J=4.86 Hz, 5.19 Hz, H₂′), 5.18(br, 1H, OH), 5.35 (br, 1H, OH), 5.73 (br, 1H, OH), 6.08 (d, 1H, J=5.11Hz, H₁′), 8.65 (s, 1H, H₈).

Step B:7-(2,3,5-Tri-O-acétyl-β-D-ribofuranosyl)imidazo[4,5-d]-ν-triazin-4-one

The compound from Step A (1.68 gr, 6.24 mmol) was stirred in pyridine(20 mL). The anhydride acetic (2.3 ml , 25 mmol) was added and themixture was stirred at 20° C. for 16 hours. The mixture was evaporatedto dryness to give a syrup which was dissolved in water. The aqueouslayer was extracted by acetyl acetate. The organic layer was evaporatedto dryness to give the title compound (1.5 gr) as a brown foam.

¹H NMR (DMSO-d₆) δ ppm: 2.04 (s, 3H, COCH₃), 2.09 (s, 3H, COCH₃), 2.10(s, 3H, COCH₃), 4.39 (m, 3H, 2×H₅′ et H₄′), 5.53 (dd, 1H, J=4.39 Hz, 5.4Hz, H₃′), 5.80 (t, 1H, J=5.4 Hz, H₂′), 6.26 (d, 1H, J=5,4 Hz, H1′), 8.15(s, 1H, H₈).

Example 13 Alternative Methods for Ribofuranosyl-Purine AnaloguesSynthesis I. Preparation of4-methylamino-7-(β-D-ribofuranosyl)imidazo[4,5-d]-ν-triazine

The 4-methylamino-7-(β-D-ribofuranosyl)imidazo[4,5-d]-ν-triazine Va maybe prepared according the following synthesis, where the startingmaterial used is the AICAR I. The AICAR may be prepared according to thepublished synthesis of Y. Yamamoto and N. Kohyama, Synthesis, 2003,17:2639-2646.

The other synthesis of4-methylamino-7-(β-D-ribofuranosyl)imidazo[4,5-d]-ν-triazine Va wasdescribed from 2-azainosine II according to the published synthesis ofL. Towsend and Co, Nucleosides, Nucleotides & Nucleic Acids, 2000,19(1&2):39-68.

II. Preparation of4-substituted-7-(2,3-dideoxy-β-D-glycero-pentofuranosyl)-imidazo-[4,5-d]-ν-triazineDerivative Compounds

The4-substituted-7-(2,3-dideoxy-β-D-glycero-pentofuranosyl)imidazo[4,5-d]-ν-triazinecompounds IXa, IXb and IXc may be prepared according the followingsynthesis according to the published synthesis of R. Panzica and Co,Bioorganic & Medicinal Chemistry, 1999, 7:2373-2379.

III. Preparation of4-substituted-7-(2,3-dideoxy-β-D-glycero-pent-2-ene-furanosyl)-imidazo-[4,5-d]-ν-triazineDerivative Compounds

The4-substituted-7-(2,3-dideoxy-β-D-glycero-pent-2-ene-furanosyl)imidazo[4,5-d]-ν-triazinederivative compounds XIa, XIb and XIc may be prepared according thefollowing synthesis:

Example 14 Phosphorylation Assay of Nucleoside to Active Triphosphate

To determine the cellular metabolism of the compounds, HepG2 cells areobtained from the American Type Culture Collection (Rockville, Md.), andare grown in 225 cm² tissue culture flasks in minimal essential mediumsupplemented with non-essential amino acids, 1% penicillin-streptomycin.The medium is renewed every three days, and the cells are subculturedonce a week. After detachment of the adherent monolayer with a 10 minuteexposure to 30 mL of trypsin-EDTA and three consecutive washes withmedium, confluent HepG2 cells are seeded at a density of 2.5×10⁶ cellsper well in a 6-well plate and exposed to 10 μM of [³H] labeled activecompound (500 dpm/pmol) for the specified time periods. The cells aremaintained at 37° C. under a 5% CO₂ atmosphere. At the selected timepoints, the cells are washed three times with ice-coldphosphate-buffered saline (PS). Intracellular active compound and itsrespective metabolites are extracted by incubating the cell pelletovernight at −20° C. with 60% methanol followed by extraction with anadditional 20 μL of cold methanol for one hour in an ice bath. Theextracts are then combined, dried under gentle filtered air flow andstored at −20° C. until HPLC analysis.

Example 15 Bioavailability Assay in Cynomolgus Monkeys

Within 1 week prior to the study initiation, the cynomolgus monkey issurgically implanted with a chronic venous catheter and sucutaneousvenous access port (VAP) to facilitate lood collection and underwent aphysical examination including hematology and serum chemistryevaluations and the body weight was recorded. Each monkey (six total)receives approximately 250 μCi of ³H activity with each dose of activecompound at a dose level of 10 mg/kg at a dose concentration of 5 mg/mL,either via an intravenous olus (3 monkeys, IV), or via oral gavage (3monkeys, PO). Each dosing syringe is weighed efore dosing togravimetrically determine the quantity of formulation administered.Urine samples are collected via pan catch at the designated intervals(approximately 18-0 hours pre-dose, 0-4, 4-8 and 8-12 hours post-dosage)and processed. blood samples are collected as well (pre-dose, 0.25, 0.5,1, 2, 3, 6, 8, 12 and 24 hours post-dosage) via the chronic venouscatheter and VAP or from a peripheral vessel if the chronic venouscatheter procedure should not be possible. The blood and urine samplesare analyzed for the maximum concentration (C_(max)), time when themaximum concentration is achieved (T_(max)), area under the curve (AUC),half life of the dosage concentration (T_(1/2)), clearance (CL), steadystate volume and distribution (V_(SS)) and bioavailability (F).

Example 16 Bone Marrow Toxicity Assay

Human one marrow cells are collected from normal healthy volunteers andthe mononuclear population are separated by Ficoll-Hypaque gradientcentrifugation as described previously by Sommadossi J-P, Carlisle R.“Toxicity of 3′-azido-3′-deoxythymidine and9-(1,3-dihydroxy-2-propoxymethyl)guanine for normal human hematopoieticprogenitor cells in vitro” Antimicrobial Agents and Chemotherapy 1987;31:452-454; and Sommadossi J-P, Schinazi R F, Chu C K, Xie M-Y.“Comparison of cytotoxicity of the (−)- and (+)-enantiomer of2′,3′-dideoxy-3′-thiacytidine in normal human one marrow progenitorcells” Biochemical Pharmacology 1992; 44:1921-1925. The culture assaysfor CFU-GM and FU-E are performed using a bilayer soft agar ormethylcellulose method. Drugs are diluted in tissue culture medium andfiltered. After 14 to 18 days at 37° C. in a humidified atmosphere of 5%CO₂ in air, colonies of greater than 50 cells are counted using aninverted microscope. The results are presented as the percent inhiitionof colony formation in the presence of drug compared to solvent controlcultures.

Example 17 Mitochondria Toxicity Assay

HepG2 cells are cultured in 12-well plates as described above andexposed to various concentrations of drugs as taught by Pan-Zhou X-R,Cui L, Zhou X-J, Sommadossi J-P, Darley-Usmer V M. “Differential effectsof antiretroviral nucleoside analogs on mitochondrial function in HepG2cells” Antimicro Agents Chemother 2000; 44:496-503. Lactic acid levelsin the culture medium after 4 day drug exposure are measured using aBoehringer lactic acid assay kit. Lactic acid levels are normalized bycell number as measured by hemocytometer count.

Example 18 Cytotoxicity Assay

Cells are seeded at a rate of between 5×10³ and 5×10⁴/well into 96-wellplates in growth medium overnight at 37° C. in a humidified CO₂ (5%)atmosphere. New growth medium containing serial dilutions of the drugsis then added. After incubation 5 for 4 days, cultures are fixed in 50%TCA and stained with sulforhodamine. The optical density was read at 550nm. The cytotoxic concentration was expressed as the concentrationrequired to reduce the cell numer by 50% (CC₅₀). The preliminary resultsare tabulated in the Table 3 below. TABLE 3 MDK versus Human HepatomaCC₅₀, μM Compound MDK β-D-4′-CH₃-riboG >250 β-D-4′-CH₃-ribo-4-thioU >250β-D-4′-CH₃-riboC >250 β-D-4′-CH₃-ribo-5-fluoroU >167β-D-4′-CH₃-riboT >250 β-D-4′-CH₃-riboA >250

This invention has been described with reference to its preferredembodiments. Variations and modifications of the invention will beobvious to those skilled in the art from the foregoing detaileddescription of the invention. It is intended that all of thesevariations and modifications be included within the scope of thisinvention.

1. A method of treating a host infected with a flavivirus or pestivirus,comprising administering an effective amount of an anti-pestivirus oranti-flavivirus biologically active ribofuranonucleoside of Formula (I):

or a pharmacologically acceptable salt or prodrug thereof, wherein: R isH, mono-, di-, or triphosphate, a stabilized phosphate, or phosphonate;X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or C-(halogen)₂,wherein alkyl, alkenyl or alkynyl may optionally be substituted; n is0-2; such than when X is CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, CH-halogen, or C-(halogen)₂, then each R¹and R^(1′) is independently H, OH, optionally substituted alkyl, loweralkyl, azido, cyano, optionally substituted alkenyl or alkynyl,—C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),—O(acyl), —O(lower acyl), —O(alkyl), —O(lower alkyl), —O(alkenyl),—O(alkynyl), halogen, halogenated alkyl, —NO₂, —NH₂, —NH(lower alkyl),—N(lower alkyl)₂, —NH(acyl), —N(acyl)₂, —C(O)NH₂, —C(O)NH(alkyl),—C(O)N(alkyl)₂, S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen,wherein alkyl, alkenyl, and/or alkynyl may optionally be substituted;and such that when X is O, S[O]_(n), NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, or SCH-halogen, then each R¹and R¹⁺ is independently H, optionally substituted alkyl, lower alkyl,azido, cyano, optionally substituted alkenyl or alkynyl, —C(O)O-(alkyl),—C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl), halogenatedalkyl, —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, —C(H)═N—NH₂, C(S)NH₂,C(S)NH(alkyl), or C(S)N(alkyl)₂, wherein alkyl, alkenyl and/or alkynylmay optionally be substituted; each R² and R³ independently is OH, NH₂,SH, F, Cl, Br, I, CN, NO₂, —C(O)NH₂, —C(O)NH(alkyl), and —C(O)N(alkyl)₂,N₃, optionally substituted alkyl, lower alkyl, optionally substitutedalkenyl or alkynyl, halogenated alkyl, —C(O)O-(alkyl), —C(O)O(loweralkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl), —O(acyl), —O(alkyl),—O(alkenyl), —O(alkynyl), —OC(O)NH₂, NC, C(O)OH, SCN, OCN, —S(alkyl),—S(alkenyl), —S(alkynyl), —NH(alkyl), —N(alkyl)₂, —NH(alkenyl),—NH(alkynyl), an amino acid residue or derivative, a prodrug or leavinggroup that provides OH in vivo, or an optionally substituted 3-7membered heterocyclic ring having O, S and/or N independently as aheteroatom taken alone or in combination; each R^(2′) and R^(3′)independently is H; optionally substituted alkyl, alkenyl, or alkynyl;—C(O)O(alkyl), —C(O)O(lower alkyl), —C(O)O(alkenyl), —C(O)O(alkynyl),—C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, —O(acyl), —O(lower acyl),—O(alkyl), —O(lower alkyl), —O(alkenyl), halogen, halogenated alkyl andparticularly CF₃, azido, cyano, NO₂, —S(alkyl), —S(alkenyl),—S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂, —NH(alkenyl), —NH(alkynyl),—NH(acyl), or —N(acyl)₂, and R₃ at 3′-C may also be OH; and Base isselected from the group consisting of:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,CN₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂;with the caveat that when X is S, then the compound is not5-(4-amino-imidazo[4,5-d][1,2,3]triazin-7-yl)-2-hydroxymethyl-tetrahydro-thiophen-3-olor7-(4-hydroxy-5-hydroxy-methyl-tetrahydro-thiophen-2-yl)-3,7-dihydro-imidazo[4,5-d][1,2,3]triazin-4-one.2. A method of treating a host infected with a flavivirus or pestivirus,comprising administering an effective amount of an anti-pestivirus oranti-flavivirus biologically active ribofuranonucleoside of Formula(II):

or a pharmacologically acceptable salt or prodrug thereof, wherein: X*is CY³; Y³ is hydrogen, alkyl, bromo, chloro, fluoro, iodo, azido,cyano, alkenyl, alkynyl, —C(O)O(alkyl), —C(O)O(lower alkyl), CF₃,—CONH₂, —CONH(alkyl), or —CON(alkyl)₂; R is H, mono-, di-, ortriphosphate, a stabilized phosphate, or phosphonate; R¹ is H, OH,optionally substituted alkyl, lower alkyl, azido, cyano, optionallysubstituted alkenyl or alkynyl. —C(O)O-(alkyl), —C(O)O(lower alkyl),—C(O)O-(alkenyl), —C(O)O-(alkynyl), —O(acyl), —O(lower acyl), —O(alkyl),—O(lower alkyl), —O(alkenyl), —O(alkynyl), halogen, halogenated alkyl,—NO₂, —NH₂, —NH(lower alkyl), —N(lower alkyl)₂, —NH(acyl), —N(acyl)₂,—C(O)NH₂, —C(O)NH(alkyl), or —C(O)N(alkyl)₂, wherein an optionalsubstitution on alkyl, alkenyl, and/or alkynyl may be one or morehalogen, hydroxy, alkoxy or alkylthio groups taken in any combination;each R² and R³ independently is OH, NH₂, F, Cl, Br, I, CN, NO₂,—C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, N₃, optionally substitutedalkyl, lower alkyl, optionally substituted alkenyl or alkynyl,halogenated alkyl, —C(O)O-(alkyl), —C(O)O(lower alkyl),—C(O)O-(alkenyl), —C(O)O-(alkynyl), an amino acid residue or derivative,a prodrug or leaving group that provides OH in vivo, or an optionallysubstituted 3-7 membered heterocyclic ring having O, S and/or Nindependently as a heteroatom taken alone or in combination; each R^(2′)and R^(3′) independently is H; optionally substituted alkyl, alkenyl, oralkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl), —C(O)O(alkenyl),—C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl), and —C(O)N(alkyl)₂, —O(acyl),—O(lower acyl), —O(alkyl), —O(lower alkyl), —O(alkenyl), halogen,halogenated alkyl and particularly CF₃, azido, cyano, NO₂, —S(alkyl),—S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂, —NH(alkenyl),—NH(alkynyl), —NH(acyl), or —N(acyl)₂, and R₃ at 3′-C may also be OH;and Base is selected from the group consisting of:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂. 3.A method of treating a host infected with a flavivirus or pestivirus,comprising administering an effective amount of an anti-pestivirus oranti-flavivirus biologically active ribofuranonucleoside of Formula(III):

or a pharmacologically acceptable salt or prodrug thereof, wherein: eachR, R^(2*), and R^(3*) independently is H, mono-, di-, or triphosphate, astabilized phosphate, or phosphonate; optionally substituted alkyl,lower alkyl, optionally substituted alkenyl or alkynyl, acyl,—C(O)-(alkyl), —C(O)(lower alkyl), —C(O)-(alkenyl), —C(O)-(alkynyl),lipid, phospholipid, carbohydrate, peptide, cholesterol, an amino acidresidue or derivative, or other pharmaceutically acceptable leavinggroup that is capable of providing H or phosphate when administered invivo; X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or C-(halogen)₂,wherein alkyl, alkenyl or alkynyl optionally may be substituted; n is0-2; each R^(2′) independently is H; optionally substituted alkyl,alkenyl, or alkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl),—C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl),—C(O)N(alkyl)₂, —OH, —O(acyl), —O(lower acyl), —O(alkyl), —O(loweralkyl), —O(alkenyl), halogen, halogenated alkyl and particularly CF₃,azido, cyano, NO₂, —S(alkyl), —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl),—N(alkyl)₂, —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂; andBase is selected from the group consisting of:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂. 4.The method of claim 3, wherein R^(2′) is an optionally substitutedalkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃, CF₃, azido,or cyano.
 5. The method of claim 3, wherein R^(2′) is an optionallysubstituted alkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃,or CF₃.
 6. The method of claim 3, wherein R^(2′) is CH₃ or CF₃.
 7. Themethod of claim 3, wherein each R, R²*, and R³* is independently H,mono-, di-, or triphosphate, a stabilized phosphate, or phosphonate. 8.The method of claim 3, wherein each R, R²*, and R³* is independently H.9. The method of claim 3, wherein each R, R²*, and R³* is independentlyH, acyl, or an amino acid acyl residue.
 10. The method of claim 3,wherein X is O or S.
 11. The method of claim 3, wherein X is O.
 12. Amethod of treating a host infected with a flavivirus or pestivirus,comprising administering an effective amount of an anti-pestivirus oranti-flavivirus biologically active ribofuranonucleoside of Formula(IV):

or a pharmacologically acceptable salt or prodrug thereof, wherein: eachR, R²*, and R³* independently is H, mono, di, or triphosphate, astabilized phosphate, or phosphonate; optionally substituted alkyl,lower alkyl, optionally substituted alkenyl or alkynyl, acyl,—C(O)-(alkyl), —C(O)(lower alkyl), —C(O)-(alkenyl), —C(O)-(alkynyl),lipid, phospholipid, carbohydrate, peptide, cholesterol, an amino acidresidue or derivative, or other pharmaceutically acceptable leavinggroup that is capable of providing H or phosphate when administered invivo; X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or C-(halogen)₂,wherein alkyl, alkenyl or alkynyl optionally may be substituted; n is0-2; each R^(3′) independently is H; optionally substituted alkyl,alkenyl, or alkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl),—C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl),—C(O)N(alkyl)₂, —OH, —O(acyl), —O(lower acyl), —O(alkyl), —O(loweralkyl), —O(alkenyl), halogen, halogenated alkyl and particularly CF₃,azido, cyano, NO₂, —S(alkyl), —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl),—N(alkyl)₂, —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂; andBase is selected from the group consisting of:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂. 13.The method of claim 12, wherein R^(3′) is an optionally substitutedalkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃, CF₃, azido,or cyano.
 14. The method of claim 12, wherein R^(3′) is an optionallysubstituted alkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃,or CF₃.
 15. The method of claim 12, wherein R^(3′) is CH₃ or CF₃. 16.The method of claim 12, wherein each R, R²*, and R³* is independently H,mono-, di-, or triphosphate, a stabilized phosphate, or phosphonate. 17.The method of claim 12, wherein each R, R²*, and R³* is independently H.18. The method of claim 12, wherein each R, R²*, and R³* isindependently H, acyl, or an amino acid acyl residue.
 19. The method ofclaim 12, wherein X is O or S.
 20. The method of claim 12, wherein X isO.
 21. The method of one of claims 3 or 12 wherein the host is a mammal.22. The method of claim 21, wherein the mammal is a human.
 23. Themethod of one of claims 3 or 12, further comprising administering anantivirally effective amount of the compound, or a pharmaceuticallyacceptable salt or prodrug thereof, in combination or alternation withone or more additional antivirally effective agents.
 24. The method ofclaim 23 wherein the additional antivirally effective agent is selectedfrom the group consisting of an interferon, ribavirin, an interleukin,an NS3 protease inhibitor, a cysteine protease inhibitor,phenanthrenequinone, a thiazolidine derivative, a thiazolidine and abenzanilide, a helicase inhibitor, a polymerase inhibitor, a nucleotideanalogue, gliotoxin, cerulenin, an antisense phosphorothioateoligodeoxynucleotide, an inhibitor of IRES-dependent translation, and aribozyme.
 25. The method of claim 24, wherein the additional antivirallyeffective agent is an interferon.
 26. The method of claim 25 wherein theadditional antivirally effective agent is selected from the groupconsisting of pegylated interferon alpha 2a, interferon alphacon-1,natural interferon, albuferon, interferon beta-1a, omega interferon,interferon alpha, interferon gamma, interferon tau, interferon delta andinterferon gamma-1b.
 27. The method of one of claims 3 and 12, whereinthe compound is in the form of a dosage unit.
 28. The method of claim 27wherein the dosage unit contains 50 to 1000 mg of the compound.
 29. Themethod of claim 28, wherein the said dosage unit is a tablet or capsule.30. The method of one of claims 3 or 12, wherein the compound is insubstantially pure form.
 31. The method of claim 30 wherein the compoundis at least 90% by weight of the β-D-isomer.
 32. The method of claim 30wherein the compound is at least 95% by weight of the β-D-isomer. 33.The method of claim 30 wherein the compound is at least 90% by weight ofthe β-L-isomer.
 34. The method of claim 30 wherein the compound is atleast 95% by weight of the β-L-isomer.
 35. A compound of the generalstructure of Formula (I):

or a pharmacologically acceptable salt or prodrug thereof, wherein: R isH, mono-, di-, or triphosphate, a stabilized phosphate, or phosphonate;X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or C-(halogen)₂,wherein alkyl, alkenyl or alkynyl may optionally be substituted; n is0-2; such than when X is CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, CH-halogen, or C-(halogen)₂, then each R¹and R^(1′) is independently H, OH, optionally substituted alkyl, loweralkyl, azido, cyano, optionally substituted alkenyl or alkynyl,—C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),—O(acyl), —O(lower acyl), —O(alkyl), —O(lower alkyl), —O(alkenyl),—O(alkynyl), halogen, halogenated alkyl, —NO₂, —NH₂, —NH(lower alkyl),—N(lower alkyl)₂, —NH(acyl), —N(acyl)₂, —C(O)NH₂, —C(O)NH(alkyl),—C(O)N(alkyl)₂, S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen,wherein alkyl, alkenyl, and/or alkynyl may optionally be substituted;and such that when X is O, S[O]_(n), NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, or SCH-halogen, then each R¹and R^(1′) is independently H, optionally substituted alkyl, loweralkyl, azido, cyano, optionally substituted alkenyl or alkynyl,—C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl), —C(O)O-(alkynyl),halogenated alkyl, —C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂,—C(H)═N—NH₂, C(S)NH₂, C(S)NH(alkyl), or C(S)N(alkyl)₂, wherein alkyl,alkenyl and/or alkynyl may optionally be substituted; each R² and R³independently is OH, NH₂, SH, F, Cl, Br, I, CN, NO₂, —C(O)NH₂,—C(O)NH(alkyl), and —C(O)N(alkyl)₂, N₃, optionally substituted alkyl,lower alkyl, optionally substituted alkenyl or alkynyl, halogenatedalkyl, —C(O)O-(alkyl), —C(O)O(lower alkyl), —C(O)O-(alkenyl),—C(O)O-(alkynyl), —O(acyl), —O(alkyl), —O(alkenyl), —O(alkynyl),—OC(O)NH₂, NC, C(O)OH, SCN, OCN, —S(alkyl), —S(alkenyl), —S(alkynyl),—NH(alkyl), —N(alkyl)₂, —NH(alkenyl), —NH(alkynyl), an amino acidresidue or derivative, a prodrug or leaving group that provides OH invivo, or an optionally substituted 3-7 membered heterocyclic ring havingO, S and/or N independently as a heteroatom taken alone or incombination; each R^(2′) and R^(3,) independently is H; optionallysubstituted alkyl, alkenyl, or alkynyl; —C(O)O(alkyl), —C(O)O(loweralkyl), —C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl),—C(O)N(alkyl)₂, —O(acyl), —O(lower acyl), —O(alkyl), —O(lower alkyl),—O(alkenyl), halogen, halogenated alkyl and particularly CF₃, azido,cyano, NO₂, —S(alkyl), —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl),—N(alkyl)₂, —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂, and R₃at 3′-C may also be OH; and Base is selected from the group consistingof:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂;with the caveat that when X is S, then the compound is not5-(4-amino-imidazo[4,5-d][1,2,3]triazin-7-yl)-2-hydroxymethyl-tetrahydro-thiophen-3-olor7-(4-hydroxy-5-hydroxy-methyl-tetrahydro-thiophen-2-yl)-3,7-dihydro-imidazo[4,5-d][1,2,3]triazin-4-one.36. A compound of the general structure of Formula (II):

or a pharmacologically acceptable salt or prodrug thereof, wherein: X*is CY³; Y³ is hydrogen, alkyl, bromo, chloro, fluoro, iodo, azido,cyano, alkenyl, alkynyl, —C(O)O(alkyl), —C(O)O(lower alkyl), CF₃,—CONH₂, —CONH(alkyl), or —CON(alkyl)₂; R is H, mono-, di-, ortriphosphate, a stabilized phosphate, or phosphonate; R¹ is H, OH,optionally substituted alkyl, lower alkyl, azido, cyano, optionallysubstituted alkenyl or alkynyl, —C(O)O-(alkyl), —C(O)O(lower alkyl),—C(O)O-(alkenyl), —C(O)O-(alkynyl), —O(acyl), —O(lower acyl), —O(alkyl),—O(lower alkyl), —O(alkenyl), —O(alkynyl), halogen, halogenated alkyl,—NO₂, —NH₂, —NH(lower alkyl), —N(lower alkyl)₂, —NH(acyl), —N(acyl)₂,—C(O)NH₂, —C(O)NH(alkyl), or —C(O)N(alkyl)₂, wherein an optionalsubstitution on alkyl, alkenyl, and/or alkynyl may be one or morehalogen, hydroxy, alkoxy or alkylthio groups taken in any combination;each R² and R³ independently is OH, NH₂, F, Cl, Br, I, CN, NO₂,—C(O)NH₂, —C(O)NH(alkyl), —C(O)N(alkyl)₂, N₃, optionally substitutedalkyl, lower alkyl, optionally substituted alkenyl or alkynyl,halogenated alkyl, —C(O)O-(alkyl), —C(O)O(lower alkyl),—C(O)O-(alkenyl), —C(O)O-(alkynyl), an amino acid residue or derivative,a prodrug or leaving group that provides OH in vivo, or an optionallysubstituted 3-7 membered heterocyclic ring having O, S and/or Nindependently as a heteroatom taken alone or in combination; each R^(2′)and R^(3′) independently is H; optionally substituted alkyl, alkenyl, oralkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl), —C(O)O(alkenyl),—C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl), and —C(O)N(alkyl)₂, —O(acyl),—O(lower acyl), —O(alkyl), —O(lower alkyl), —O(alkenyl), halogen,halogenated alkyl and particularly CF₃, azido, cyano, NO₂, —S(alkyl),—S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl), —N(alkyl)₂, —NH(alkenyl),—NH(alkynyl), —NH(acyl), or —N(acyl)₂, and R₃ at 3′-C may also be OH;and Base is selected from the group consisting of:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂. 37.A compound of the general structure of Formula (III):

or a pharmacologically acceptable salt or prodrug thereof, wherein: eachR, R²*, and R³* independently is H, mono-, di-, or triphosphate, astabilized phosphate, or phosphonate; optionally substituted alkyl,lower alkyl, optionally substituted alkenyl or alkynyl, acyl,—C(O)-(alkyl), —C(O)(lower alkyl), —C(O)-(alkenyl), —C(O)-(alkynyl),lipid, phospholipid, carbohydrate, peptide, cholesterol, an amino acidresidue or derivative, or other pharmaceutically acceptable leavinggroup that is capable of providing H or phosphate when administered invivo; X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or C-(halogen)₂,wherein alkyl, alkenyl or alkynyl optionally may be substituted; n is0-2; each R^(2′) independently is H; optionally substituted alkyl,alkenyl, or alkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl),—C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl),—C(O)N(alkyl)₂, —OH, —O(acyl), —O(lower acyl), —O(alkyl), —O(loweralkyl), —O(alkenyl), halogen, halogenated alkyl and particularly CF₃,azido, cyano, NO₂, —S(alkyl), —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl),—N(alkyl)₂, —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂; andBase is selected from the group consisting of:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂₋₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂. 38.The compound of claim 37, wherein R^(2′) is an optionally substitutedalkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃, CF₃, azido,or cyano.
 39. The compound of claim 37, wherein R^(2′) is an optionallysubstituted alkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃,or CF₃.
 40. The compound of claim 37, wherein R^(2′) is CH₃ or CF₃. 41.The compound of claim 37, wherein each R, R²*, and R³* is independentlyH, mono-, di-, or triphosphate, a stabilized phosphate, or phosphonate.42. The compound of claim 37, wherein each R, R²*, and R³* isindependently H.
 43. The compound of claim 37, wherein each R, R²*, andR³* is independently H, acyl, or an amino acid acyl residue.
 44. Thecompound of claim 37, wherein X is O or S.
 45. The compound of claim 37,wherein X is O.
 46. A compound of the general structure of Formula (IV):

or a pharmacologically acceptable salt or prodrug thereof, wherein: eachR, R²*, and R³* independently is H, mono-, di-, or triphosphate, astabilized phosphate, or phosphonate; optionally substituted alkyl,lower alkyl, optionally substituted alkenyl or alkynyl, acyl,—C(O)-(alkyl), —C(O)(lower alkyl), —C(O)-(alkenyl), —C(O)-(alkynyl),lipid, phospholipid, carbohydrate, peptide, cholesterol, an amino acidresidue or derivative, or other pharmaceutically acceptable leavinggroup that is capable of providing H or phosphate when administered invivo; X is O, S[O]_(n), CH₂, CHOH, CH-alkyl, CH-alkenyl, CH-alkynyl,C-dialkyl, CH—O-alkyl, CH—O-alkenyl, CH—O-alkynyl, CH—S-alkyl,CH—S-alkenyl, CH—S-alkynyl, NH, N-alkyl, N-alkenyl, N-alkynyl,S(O)N-alkyl, S(O)N-alkenyl, S(O)N-alkynyl, SCH-halogen, or C-(halogen)₂,wherein alkyl, alkenyl or alkynyl optionally may be substituted; n is0-2; each R^(3′) independently is H; optionally substituted alkyl,alkenyl, or alkynyl; —C(O)O(alkyl), —C(O)O(lower alkyl),—C(O)O(alkenyl), —C(O)O(alkynyl), —C(O)NH₂, —C(O)NH(alkyl),—C(O)N(alkyl)₂, —OH, —O(acyl), —O(lower acyl), —O(alkyl), —O(loweralkyl), —O(alkenyl), halogen, halogenated alkyl and particularly CF₃,azido, cyano, NO₂, —S(alkyl), —S(alkenyl), —S(alkynyl), NH₂, —NH(alkyl),—N(alkyl)₂, —NH(alkenyl), —NH(alkynyl), —NH(acyl), or —N(acyl)₂; andBase is selected from the group consisting of:

wherein each R′ and R″ independently is H, C₁₋₆ alkyl, C₂ ₆ alkenyl,C₂₋₆ alkynyl, halogen, halogenated alkyl, OH, CN, N₃, carboxy,C₁₋₄alkoxycarbonyl, NH₂, C₁₋₄ alkylamino, di(C₁₋₄ alkyl)amino, C₁₋₆alkoxy, C₁₋₆ alkylsulfonyl, (C₁₋₄ alkyl)₀₋₂ aminomethyl; each W is Cl,Br, I, F, halogenated alkyl, alkoxy, OH, SH, O-alkyl, S-alkyl,O-alkenyl, O-alkynyl, S-alkenyl, S-alkynyl, —OC(O)NR⁴R⁴, O-acyl, S-acyl,CN, SCN, OCN, NO₂, N₃, NH₂, NH(alkyl), N(alkyl)₂, NH-cycloalkyl,NH-acyl, NH═NH, CONH₂, CONH(alkyl), or CON(alkyl)₂; and each R⁴ isindependently H, acyl, or C₁₋₆ alkyl; each Z is O, S, NH, N—OH, N—NH₂,NH(alkyl), N(alkyl)₂, N-cycloalkyl, alkoxy, CN, SCN, OCN, SH, NO₂, NH₂,N₃, NH═NH, NH(alkyl), N(alkyl)₂, CONH₂, CONH(alkyl), or CON(alkyl)₂. 47.The compound of claim 46, wherein R^(3′) is an optionally substitutedalkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃, CF₃, azido,or cyano.
 48. The compound of claim 46, wherein R^(3′) is an optionallysubstituted alkyl, alkenyl, or alkynyl; halogen, halogenated alkyl, CH₃,or CF₃.
 49. The compound of claim 46, wherein R^(3′) is CH₃ or CF₃. 50.The compound of claim 46, wherein each R, R²*, and R³* is independentlyH, mono-, di-, or triphosphate, a stabilized phosphate, or phosphonate.51. The compound of claim 46, wherein each R, R²*, and R³* isindependently H.
 52. The compound of claim 46, wherein each R, R²*, andR³* is independently H, acyl, or an amino acid acyl residue.
 53. Thecompound of claim 46, wherein X is O or S.
 54. The compound of claim 46,wherein X is O.
 55. A pharmaceutical composition comprising ananti-virally effective amount of a compound of one of claims 37 or 46,optionally with a pharmaceutically acceptable carrier, diluent orexcipient.
 56. The pharmaceutical composition of claim 55 wherein thecompound, salt or prodrug thereof is in the form of a dosage unit. 57.The pharmaceutical composition of claim 56 wherein the dosage unitcontains from about 0.01 to about 50 mg of the compound.
 58. Thepharmaceutical composition of claim 57, wherein said dosage unit is atablet or capsule.
 59. The pharmaceutical composition of claim 55,further comprising one or more additional anti-virally effective agents.60. The pharmaceutical composition of claim 59, wherein the additionalanti-virally agent is selected from the group consisting of aninterferon, ribavirin, an interleukin, an NS3 protease inhibitor, acysteine protease inhibitor, a thiazolidine derivative, a thiazolidineand a benzanilide, phenanthrenequinone, a helicase inhibitor, apolymerase inhibitor, a nucleotide analogue, gliotoxin, cerulenin, anantisense oligodeoxynucleotide, an inhibitor of IRES-dependenttranslation, and a ribozyme.
 61. The pharmaceutical composition of claim60 wherein the additional anti-virally effective agent is an interferon.62. The pharmaceutical composition of claim 61, wherein the additionalanti-virally effective agent is selected from the group consisting ofpegylated interferon alpha 2a, interferon alphacon-1, naturalinterferon, albuferon, interferon beta-1a, omega interferon, interferonalpha, interferon gamma, interferon tau, interferon delta and interferongamma-1b.
 63. The pharmaceutical composition of one of claims 37 or 46,wherein the compound is in substantially pure form.
 64. Thepharmaceutical composition of claim 63, wherein the compound is at least90% by weight of the β-D-isomer.
 65. The pharmaceutical composition ofclaim 63, wherein the compound is at least 95% by weight of theβ-D-isomer.
 66. The pharmaceutical composition of claim 63 wherein thecompound is at least 90% by weight of the β-L-isomer.
 67. Thepharmaceutical composition of claim 63 wherein the compound is at least95% by weight of the β-L-isomer.